Protect reentrant animations when IMMEDIATELY_ANIMATE_TO_NEW_TARGET is used.

BUG=297974
TEST=TBD

Review URL: https://codereview.chromium.org/25107002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226111 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226111