Fixes a crash when copying elements with font-weight lighter or bolder

While investigating some editing crashes, I noticed an assertion
triggering in EditingStyle due to no treatment over font-weight's
'lighter' and 'bolder' values. This patch fixes this and adds a test to
verify it does not crash when copying/pasting elements using these
values.

BUG=272363

Review URL: https://chromiumcodereview.appspot.com/23075004

git-svn-id: svn://svn.chromium.org/blink/trunk@156188 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add histograms to measure webfont load latency by layout

Currently WebFont download is queued during layout and actual download is
delayed until after layout. We should be able to kick off download at
style recalc time, to reduce font load latency.

This patch adds two UMA metrics:

WebFont.LayoutLatency histogram measures how early we can start loading,
i.e. the time from RenderText::styleDidChange to CSSFontFaceSource::didStartFontLoad.

WebFont.UsageType measures how often
- a style rule matches, but the font isn't needed
- a font is used, but RenderText::styleDidChange was not called before
- a style rule matches, and the font is used

BUG=246492

Review URL: https://chromiumcodereview.appspot.com/23186004

git-svn-id: svn://svn.chromium.org/blink/trunk@156189 bbb929c8-8fbe-4397-9dbb-9b2b20218538