Try to do less work clearing the font-cache in StyleEngine::didDetach.

Clearing the whole font-cache without considering css-connected fonts
could be cheaper. This is a speculative fix for issue 677415.

BUG=677415

Review-Url: https://codereview.chromium.org/2622673003
Cr-Commit-Position: refs/heads/master@{#443202}
1 file changed