Implement CSSOM View smooth scroll for Window.{scroll, scrollTo, scrollBy}

This implements smooth scrolling for scroll(), scrollTo(), and scrollBy()
on main frames and subframes.

Smooth scrolling for Elements (that is, for setting scrollTop and scrollLeft
or calling scrollIntoView) will be added in later CLs.

For now, all scroll animations are performed on the main-thread. The use
of compositor-driven animations (for ScrollableAreas that have
compositor-driven scrolling) will be added in a later CL.

Intent-to-implement: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/YLWs9N46XjQ

Spec: http://www.w3.org/TR/cssom-view/

BUG=243871

Review URL: https://codereview.chromium.org/384973003

git-svn-id: svn://svn.chromium.org/blink/trunk@178755 bbb929c8-8fbe-4397-9dbb-9b2b20218538