Only return true for supportsControlTints on Mac.

Control tints are controls changing color when you blur/focus
a window. I believe they're also for when you change the
appearance from system preferences. In either case, it's a
Mac-only feature as best I can tell.

Review URL: https://codereview.chromium.org/164333012

git-svn-id: svn://svn.chromium.org/blink/trunk@168047 bbb929c8-8fbe-4397-9dbb-9b2b20218538