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
Forward-declare Player in CSSPendingAnimations

Trivial clean-up; reduces steps in incremental compile after touching
Player.h from about 1200 to 700.

BUG=

Review URL: https://codereview.chromium.org/142943002

git-svn-id: svn://svn.chromium.org/blink/trunk@165424 bbb929c8-8fbe-4397-9dbb-9b2b20218538