Correctly measure SVG font usage when referenced through CSS

Our previous measurement of SVGFontElement data contained a bug where
fonts referenced via CSS were not properly counted because the font
element's page is null. This patch adds a second measurement for SVG
fonts, "SVGFontInCSS", to properly measure this feature's usage.

Original patch that added SVGFontElement measurement:
https://src.chromium.org/viewvc/blink?view=rev&revision=150855

BUG=304219

Review URL: https://codereview.chromium.org/26016004

git-svn-id: svn://svn.chromium.org/blink/trunk@158980 bbb929c8-8fbe-4397-9dbb-9b2b20218538