Fix instrumentation for webfonts download

CSSFontFaceSource::getFontData() does not start loading webfonts immediately,
but schedules donwload after layout is complete.
WebFont.LoadTime.* histograms have measured this layout time + download time.
We want to separate those two, so this patch changes WebFont.LoadTime.*
to WebFont.DownloadTime.* that measure only the download duration.
The latency by layout will be measured by another UMA (crbug.com/246492).

BUG=237058

Review URL: https://chromiumcodereview.appspot.com/22369002

git-svn-id: svn://svn.chromium.org/blink/trunk@155601 bbb929c8-8fbe-4397-9dbb-9b2b20218538