Web Animations API: Add Animation Constructor to API

ElementAnimation::animate now calls the Animation constructor, so most of the logic from ElementAnimation is now in Animation.

ElementAnimationTest.cpp moved to AnimationTest.cpp (as bypassing document/element status checks was going to get messy).

BUG=327564,334932

Review URL: https://codereview.chromium.org/150063002

git-svn-id: svn://svn.chromium.org/blink/trunk@166496 bbb929c8-8fbe-4397-9dbb-9b2b20218538