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