Web Animations: Add bindings for Player

This patch adds the remaining bindings for Player, aside from timeline
since we haven't yet added the DocumentTimeline bindings. Since most
of the functionality of Player is unit tested, the added test only
does sanity checks for the main parts of the interface. Also this makes
the element.animate test stop using internals and instead use the added
bindings.

Spec: http://dev.w3.org/fxtf/web-animations/#idl-def-Player

BUG=334926

Review URL: https://codereview.chromium.org/152493002

git-svn-id: svn://svn.chromium.org/blink/trunk@166457 bbb929c8-8fbe-4397-9dbb-9b2b20218538