Manage RequestAnimationFrameCallbacks with OwnPtrs instead of RefPtrs

Blink now prefers non-refcounted IDL callbacks. RAF slightly abused the refcounted-ness
of its callback, which this patch takes care of removing.

It also allows simplifies the m_firedOrCancelled member to just m_cancelled.

R=jamesr@chromium.org
BUG=323681

Review URL: https://codereview.chromium.org/92913002

git-svn-id: svn://svn.chromium.org/blink/trunk@163026 bbb929c8-8fbe-4397-9dbb-9b2b20218538