Expose GL_EXT_multisampled_render_to_texture whenever it's available.

We allow using the extension even when GL(ES)3-type MSAA is present
(these two are not exactly the same thing; since the extension uses
an implicit resolve step, it allows better performance on some GPUs).

BUG=322706

Review URL: https://codereview.chromium.org/115733005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241487 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241487