WebFont.BlankTextShownTime should not include the time fallback text is displayed after 3 seconds

WebFont.BlankTextShownTime was measuring the time until font load, but
that is not really the time user sees blank text because we introduced
the 3 second timeout to show fallback text (crbug.com/235303).
So we should call FontLoadHistograms::recordFallbackTime() when the
3 seconds timeout is kicked, too.

BUG=345839
TEST=Manually tested with chrome://histograms

Review URL: https://codereview.chromium.org/193443005

git-svn-id: svn://svn.chromium.org/blink/trunk@168977 bbb929c8-8fbe-4397-9dbb-9b2b20218538