Disable experimental subpixel font scaling on mac

The experimental subpixel font scaling feature made a couple of SVG
text test flaky on Mac. Temporarily disable it while investigating.

BUG=324553
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/97953002

git-svn-id: svn://svn.chromium.org/blink/trunk@162948 bbb929c8-8fbe-4397-9dbb-9b2b20218538