Web Animations CSS: Ensure player is cancelled when compositor transition is retargetted

An error in r162743 [1] meant that when a compositor animation was retargetted
the old player was never cancelled. This meant that every time a subsequent
transition finished the forwards fill from the old player was reinstated
causing futher transitions to begin.

[1] http://src.chromium.org/viewvc/blink?view=rev&revision=162743

BUG=325348

Review URL: https://codereview.chromium.org/102883002

git-svn-id: svn://svn.chromium.org/blink/trunk@163120 bbb929c8-8fbe-4397-9dbb-9b2b20218538