Remove redundant calls to scheduleAnimation

This CL removes a number of redundant calls to scheduleAnimation. Now we track
in PageAnimator whether we've already schedule an animation frame.

This CL is another small step towards having PageAnimator drive the
per-animation frame work.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/177113003

git-svn-id: svn://svn.chromium.org/blink/trunk@167775 bbb929c8-8fbe-4397-9dbb-9b2b20218538