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The function is called ~20,000 times on a fresh launch of Chrome on my MacBook
Pro. Adding a dumb cache to reduce redundant fetches reduced cpu cycles in the
function by 75% (profiled by DTrace).

The class FontFamilyCache performs the caching, and the observation of the
PrefService to ensure that the cache does not become stale.

The Profile owns the FontFamilyCache to ensure that the cache does not
outlive the PrefService.

BUG=308095

Review URL: https://codereview.chromium.org/439913002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287953 0039d316-1c4b-4281-b951-d872f2087c98