ui: message_center and app_list tests opt into real GL NullDraw contexts

This breaks the dependency on the TestCompositor for
these test suites. Once it's no longer in use by any test
suite, we can remove it.

These tests only compile in the compositor and ui/gl/ on non-Mac
platforms, so only initialize GL there.

Depends on: https://codereview.chromium.org/175093004/

R=ben@chromium.org
BUG=270918

Review URL: https://codereview.chromium.org/168193003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252771 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252771