Move webfont usage metrics reporting to FontResource

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=157280

[Kunihiko Sakamoto, 2013-09-05 04:42:59]

Kent-san, could you take a look?

[tkent, 2013-09-05 04:57:08]

lgtm

[Kunihiko Sakamoto, 2013-09-05 05:05:22]

On 2013/09/05 04:57:08, tkent wrote:
> lgtm

Thanks. I'm going to submit this after getting lgtm for histograms.xml.

[commit-bot: I haz the power, 2013-09-05 05:39:28]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ksakamoto@chromium.org/23720005/1

[commit-bot: I haz the power, 2013-09-05 06:49:41]

Change committed as 157280