Enable subpixel font scaling by default

Allow sub-css-pixel font-sizes and thus smooth font scaling which in
turn allows for stable layout when zooming and drastically improves
readability and font rendering quality on high dpi devices.

This change enabled the experimental subpixel font scaling flag by
default across all supported platforms.

Entry in Chromium Dashboard
  http://www.chromestatus.com/features/6259163214643200

OWP launch tracking bug
  https://code.google.com/p/chromium/issues/detail?id=164167

Intent to ship
  https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/4-f2i7OEWeg

R=eseidel@chromium.org, dglazkov@chromium.org
BUG=164167

Review URL: https://codereview.chromium.org/307993009

git-svn-id: svn://svn.chromium.org/blink/trunk@175192 bbb929c8-8fbe-4397-9dbb-9b2b20218538