Enable the threaded-compositing command line switches even under Aura.

Previously we had an #ifdef AURA that was overriding things and would
break running the layout tests under Aura.

R=jamesr@chromium.org, piman@chromium.org
BUG=229207, 233830

Review URL: https://codereview.chromium.org/56243004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232546 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232546