Fix flake in LayerTreeHostAnimationTestFrozenAnimationTickTime

This changes the test to make sure it doesn't expect animations to
be frozen after EndTest(), since after this point we allow draws to
succeed.

BUG=344078

Review URL: https://codereview.chromium.org/170593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251813 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251813