Add a UMA histogram for type of web font usage

This patch adds WebFont.PackageFormat histogram that tracks the usage of
different format of web fonts. It records container format of font file;
both TrueType fonts and OpenType fonts are recorded as SFNT format.
If a font didn't pass OTS sanitization, it's recoreded as Unknown.

BUG=380988
TEST=manually tested with chrome://histograms

Review URL: https://codereview.chromium.org/324003002

git-svn-id: svn://svn.chromium.org/blink/trunk@175991 bbb929c8-8fbe-4397-9dbb-9b2b20218538