UMA histograms to measure whether / how long blank text is rendered for loading web fonts

This patch adds two UMA metrics:

WebFont.BlankTextShownTime histogram tracks the time we spent showing
blank text. This measures the time between when the "loading fallback"
font is drawn for the first time, and when FontData for the downloaded
web font is created.

WebFont.HadBlankText is recorded once per page load if the page uses
any web fonts. The value is 1 if we ended up showing blank text at
least once, 0 otherwise.

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

Review URL: https://codereview.chromium.org/180273020

git-svn-id: svn://svn.chromium.org/blink/trunk@168541 bbb929c8-8fbe-4397-9dbb-9b2b20218538