Oilpan: fix failing animation unit tests.

Address three failures:

 * AnimationTest.ElementDestructorClearsAnimationTarget
   Animation objects now die with their target element, hence testing
   for it separately being destructed doesn't make sense in an
   Oilpan setting.
 * AnimationStackTest.ForwardsFillDiscarding
   In order to be able to accurately sample the number of effects on
   the stack, explicitly GC before querying.
 * AnimationPlayerTest.AttachedAnimationPlayers
   Use a persistent to correctly hold on to the test element. If not,
   the final GC will destruct it also & making any further testing
   over it invalid.

R=haraken@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/287343003

git-svn-id: svn://svn.chromium.org/blink/trunk@174261 bbb929c8-8fbe-4397-9dbb-9b2b20218538