A fix for flash of unstyled font

[natashapl]

This Pull request will include the following:

• A possible fix for the flash of unstyled font issue.

PR Preview

[ethangardner]

Here is before/after with font caching and a preload. I preloaded the heading font only, since that seems to be the most noticeable cause of FOUT. In the tests below, the before is labeled "baseline fout" and the after is "with font preload and cache test."

This is the first view: https://www.webpagetest.org/video/compare.php?tests=240626_BiDcB7_BCV%2C240626_BiDcD8_BD1&thumbSize=200&ival=100&end=visual

FCP is quicker in the before version, but LCP ends up being quicker in the after.

Repeat view: https://www.webpagetest.org/video/compare.php?tests=240626_BiDcB7_BCV-c:1%2C240626_BiDcD8_BD1-c:1&thumbSize=200&ival=100&end=visual

Repeat views will have the most pronounced impact since the fonts will be in cache and don't need to get fetched from the network. In all cases, FOUT from the heading seems to be improved. Ready for review.

[jonadecker]

💯

[jonadecker]

@natashapl / @ethangardner can you please resolve the conflicts on this branch?