Enable experimental support for sub-pixel font scaling

Enable sub-css-pixel font scaling as an experimental web feature.
This would allow for things like font-size: 12.5px which would map to
25 device pixels on a high dpi display or when zoomed in.
It also allows for smoother text scaling and consistent sizing across
zoom levels.

R=dglazkov@chromium.org
BUG=290411
TEST=fast/text/sub-pixel/text-scaling*

Review URL: https://codereview.chromium.org/25512005

git-svn-id: svn://svn.chromium.org/blink/trunk@162893 bbb929c8-8fbe-4397-9dbb-9b2b20218538