Add a VirtualTestSuite for fast text autosizing.

This ensures the text autosizing tests run with the --enable-fast-text-autosizing flag (and allows us to easily set other flags for them in the future).

BUG=302005

Review URL: https://codereview.chromium.org/25649007

git-svn-id: svn://svn.chromium.org/blink/trunk@159418 bbb929c8-8fbe-4397-9dbb-9b2b20218538