Add support for WebFont sub-css-pixel font sizes

Add support for sub-css-pixel font sizes for web fonts when the
SubpixelFontScaling feature is enabled.

Covered by http://blink.gs/tests/text-scaling/webfont.html, will add
as a LayoutTest once supported on all platforms.

R=leviw@chromium.org
BUG=290411

Review URL: https://codereview.chromium.org/25638008

git-svn-id: svn://svn.chromium.org/blink/trunk@158945 bbb929c8-8fbe-4397-9dbb-9b2b20218538