enable stencil buffer on mac; allows --force-direct-layer-drawing to work

BUG=

Review URL: https://codereview.chromium.org/27212002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234868 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234868