Reenable MSAA on Mali

Now that we have a WebGL implementation of MSAA using
EXT_multisampled_render_to_texture, we can reenable MSAA on Mali GPUs.

BUG=322706
TEST=run on daisy, WebGL now uses multisampling and simple demos run at 60 fps.

Review URL: https://codereview.chromium.org/188173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255601 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255601