Add a flag for fast text autosizing

This is the third of three patches to add a runtime flag for fast text
autosizing. This patch adds the Content-side plumbing to enable
the Blink-side feature.

This patch depends on https://codereview.chromium.org/25452002 landing.

BUG=302005

Review URL: https://codereview.chromium.org/25454002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226525 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226525