Add a runtime flag for fast text autosizing

This is the first of 3 patches to add a runtime flag for a fast text
autosizing implementation. This patch adds the off-by-default Blink-side
flag which we will flip on using a subsequent Content-side patch.

BUG=302005

Review URL: https://codereview.chromium.org/25314004

git-svn-id: svn://svn.chromium.org/blink/trunk@158570 bbb929c8-8fbe-4397-9dbb-9b2b20218538