Add WebFont.HadBlankText and WebFont.BlankTextShownTime histograms

The Blink-side change is here: https://codereview.chromium.org/180273020/

BUG=345839
NOTRY=true

Review URL: https://codereview.chromium.org/183743016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255225 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255225