Web Animations: Add trivial bindings for Animation, Player, TimedItem and make Element.animate return an Animation

This patch adds trivial bindings for exposing the Web Animations classes
Animation, Player and TimedItem. It also makes element.animate return an
Animation object as per the Animatable interface. The new interfaces are
obviously incomplete, missing most methods and attributes. Note that the
source attribute on Player is currently marked as readonly while it will
later need to have this modifier removed.

BUG=334932,334927,334926,327564

Review URL: https://codereview.chromium.org/140623003

git-svn-id: svn://svn.chromium.org/blink/trunk@165423 bbb929c8-8fbe-4397-9dbb-9b2b20218538