enable antialiasing for skia-gpu

This fixes hulu.com (yay!) and does not seem to slow down these sites:
- fishtank
- speedreading
- galaxy

Note -- I ran DRT locally, but I'm not seeing the failures I would expect... :(

Here's what I ran

    ./third_party/WebKit/Tools/Scripts/new-run-webkit-tests --platform chromium-gpu
Review URL: http://codereview.chromium.org/7058070

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@87999 0039d316-1c4b-4281-b951-d872f2087c98