Animations: Allow document animation timelines to have a playback rate

This change allows for animation timelines to have a playback rate
specified which can then be used to pause or slow down all animations
on a timeline. An inspector protocol method is also added to allow the
animation timeline playback rate to be set on all document animation
timelines associated with a page.

BUG=432368

Review URL: https://codereview.chromium.org/717003002

git-svn-id: svn://svn.chromium.org/blink/trunk@185262 bbb929c8-8fbe-4397-9dbb-9b2b20218538