mac: Disable CoreAnimation throbbers for now, while I investigate the performance regressions.

BUG=345657

Review URL: https://codereview.chromium.org/174883003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252819 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252819