Switch AutoscrollController to use animation system instead of timer

This switches the AutoscrollController from using a 50ms timer to using
our animation system to do pan scrolling. The 50ms timer capped us at a janky
20FPS at best whereas our animation system can easily hit 60FPS in most cases.
This also cleans up the controller logic a bit to thin down the Page interface.

BUG=310791

Review URL: https://codereview.chromium.org/46353003

git-svn-id: svn://svn.chromium.org/blink/trunk@161201 bbb929c8-8fbe-4397-9dbb-9b2b20218538