Add more WebRuntimeFeatures plumbing for fast text autosizing

This is the second of 3 patches to add a runtime flag for a fast text
autosizing implementation. This patch adds additional plumbing to
WebRuntimeFeatures that was left out of
http://src.chromium.org/viewvc/blink?view=rev&rev=158570. A subsequent
Content-side patch will add a command-line flag to call this.

Once this lands, we can land https://codereview.chromium.org/25454002

BUG=302005

Review URL: https://codereview.chromium.org/25452002

git-svn-id: svn://svn.chromium.org/blink/trunk@158650 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Removing my old email from watchlist.

BUG=

Review URL: https://codereview.chromium.org/25568002

git-svn-id: svn://svn.chromium.org/blink/trunk@158651 bbb929c8-8fbe-4397-9dbb-9b2b20218538