Move webfont usage metrics reporting to FontResource

CSSFontFaceSource::FontLoadHistograms class reported WebFont.UsageType
and WebFont.StyleRecalcToDownloadLatency metrics. But CSSFontFaceSource
is destructed/reconstructed when a new @font-face is added to the
stylesheet, so these metrics were reported more than once for single
webfont when the web page dynamically generates @font-face rules.
That caused unexpectedly high "Styled, but not used" count of
WebFont.UsageType.

To avoid this, this patch moves the reporting code to a inner class of
FontResource, and rename the histogram names to WebFont.Resource.*.
The metrics are recorded at most once for each FontResource object
which is retained by the memory cache.

BUG=246492

Review URL: https://chromiumcodereview.appspot.com/23720005

git-svn-id: svn://svn.chromium.org/blink/trunk@157280 bbb929c8-8fbe-4397-9dbb-9b2b20218538