Don't drop scheduleAnimation when there is no layer tree

isAcceleratedCompositingActive is only true when a compositing update has
happened with a non-null layer tree and so is a racy setting.

This is danakj's suggestion from https://crbug.com/393331#c60.

BUG=393331

Review URL: https://codereview.chromium.org/595083002

git-svn-id: svn://svn.chromium.org/blink/trunk@182621 bbb929c8-8fbe-4397-9dbb-9b2b20218538