Oilpan: Fix AnimationAnimationTest.ElementDestructorClearsAnimationTarget in webkit_unit_tests

This is a follow-up fix after r172009.
Currently AnimationAnimationTest.ElementDestructorClearsAnimationTarget is failing in oilpan bots.
We need to explicitly trigger a GC before checking that animation->target() is deleted.

NOTRY=true

Review URL: https://codereview.chromium.org/246473004

git-svn-id: svn://svn.chromium.org/blink/trunk@172116 bbb929c8-8fbe-4397-9dbb-9b2b20218538