Add histogram to measure webfont cache hit ratio

This patch adds WebFont.CacheHit histogram which measures how often
- A WebFont is loaded from cache (memory cache or disk cache)
- A WebFont is fetched from network
- A WebFont is served by data: URL

BUG=283631

Review URL: https://codereview.chromium.org/27468002

git-svn-id: svn://svn.chromium.org/blink/trunk@160056 bbb929c8-8fbe-4397-9dbb-9b2b20218538