Add WebFont.CacheHit histogram to histograms.xml

The histogram was added here https://codereview.chromium.org/27468002

BUG=283631
NOTRY=true

Review URL: https://codereview.chromium.org/31583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230051 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230051