todo-close

        /// <summary>
        /// Called when a browser has recieved a request to close. This may result
        /// directly from a call to CefBrowserHost::*CloseBrowser() or indirectly if
        /// the browser is parented to a top-level window created by CEF and the user
        /// attempts to close that window (by clicking the 'X', for example). The
        /// DoClose() method will be called after the JavaScript 'onunload' event has
        /// been fired.
        /// An application should handle top-level owner window close notifications by
        /// calling CefBrowserHost::TryCloseBrowser() or
        /// CefBrowserHost::CloseBrowser(false) instead of allowing the window to close
        /// immediately (see the examples below). This gives CEF an opportunity to
        /// process the 'onbeforeunload' event and optionally cancel the close before
        /// DoClose() is called.
        /// When windowed rendering is enabled CEF will internally create a window or
        /// view to host the browser. In that case returning false from DoClose() will
        /// send the standard close notification to the browser's top-level owner
        /// window (e.g. WM_CLOSE on Windows, performClose: on OS X, "delete_event" on
        /// Linux or CefWindowDelegate::CanClose() callback from Views). If the
        /// browser's host window/view has already been destroyed (via view hierarchy
        /// tear-down, for example) then DoClose() will not be called for that browser
        /// since is no longer possible to cancel the close.
        /// When windowed rendering is disabled returning false from DoClose() will
        /// cause the browser object to be destroyed immediately.
        /// If the browser's top-level owner window requires a non-standard close
        /// notification then send that notification from DoClose() and return true.
        /// The CefLifeSpanHandler::OnBeforeClose() method will be called after
        /// DoClose() (if DoClose() is called) and immediately before the browser
        /// object is destroyed. The application should only exit after OnBeforeClose()
        /// has been called for all existing browsers.
        /// The below examples describe what should happen during window close when the
        /// browser is parented to an application-provided top-level window.

        /// Example 1: Using CefBrowserHost::TryCloseBrowser(). This is recommended for
        /// clients using standard close handling and windows created on the browser
        /// process UI thread.
        /// 1.  User clicks the window close button which sends a close notification to
        /// the application's top-level window.
        /// 2.  Application's top-level window receives the close notification and
        /// calls TryCloseBrowser() (which internally calls CloseBrowser(false)).
        /// TryCloseBrowser() returns false so the client cancels the window close.
        /// 3.  JavaScript 'onbeforeunload' handler executes and shows the close
        /// confirmation dialog (which can be overridden via
        /// CefJSDialogHandler::OnBeforeUnloadDialog()).
        /// 4.  User approves the close.
        /// 5.  JavaScript 'onunload' handler executes.
        /// 6.  CEF sends a close notification to the application's top-level window
        /// (because DoClose() returned false by default).
        /// 7.  Application's top-level window receives the close notification and
        /// calls TryCloseBrowser(). TryCloseBrowser() returns true so the client
        /// allows the window close.
        /// 8.  Application's top-level window is destroyed.
        /// 9.  Application's OnBeforeClose() handler is called and the browser object
        /// is destroyed.
        /// 10. Application exits by calling CefQuitMessageLoop() if no other browsers
        /// exist.

        /// Example 2: Using CefBrowserHost::CloseBrowser(false) and implementing the
        /// DoClose() callback. This is recommended for clients using non-standard
        /// close handling or windows that were not created on the browser process UI
        /// thread.
        /// 1.  User clicks the window close button which sends a close notification to
        /// the application's top-level window.
        /// 2.  Application's top-level window receives the close notification and:
        /// A. Calls CefBrowserHost::CloseBrowser(false).
        /// B. Cancels the window close.
        /// 3.  JavaScript 'onbeforeunload' handler executes and shows the close
        /// confirmation dialog (which can be overridden via
        /// CefJSDialogHandler::OnBeforeUnloadDialog()).
        /// 4.  User approves the close.
        /// 5.  JavaScript 'onunload' handler executes.
        /// 6.  Application's DoClose() handler is called. Application will:
        /// A. Set a flag to indicate that the next close attempt will be allowed.
        /// B. Return false.
        /// 7.  CEF sends an close notification to the application's top-level window.
        /// 8.  Application's top-level window receives the close notification and
        /// allows the window to close based on the flag from #6B.
        /// 9.  Application's top-level window is destroyed.
        /// 10. Application's OnBeforeClose() handler is called and the browser object
        /// is destroyed.
        /// 11. Application exits by calling CefQuitMessageLoop() if no other browsers
        /// exist.
        /// </summary>