Turn on compositing for animations with software compositor.

R=jbauman@chromium.org, piman@chromium.org, jbauman, piman
BUG=350141

Review URL: https://codereview.chromium.org/191563003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255962 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255962