Web Animations CSS: Implement TransitionTimeline

This patch adds a separate timeline for CSS transitions. This fixes a
couple of problems of using the same timeline -- we can now correctly
apply transitions at the last step of the cascade, and we can also run
transitions before the document has been loaded.

This requires several changes to support it. We add a 'priority'
field to Animation to determine whether we are a CSS Transition or
other Animation (the Animations go to the same AnimationStack).
The logic for getting the compositable values to apply is moved to
the AnimationStack, and CSSAnimations determines the new/cancelled
animations. We also no longer recompute the compositable values in
each applyAnimatedProperties pass, instead doing this once for each
of transitions and animations beforehand.

BUG=303430

Review URL: https://codereview.chromium.org/27372007

git-svn-id: svn://svn.chromium.org/blink/trunk@160306 bbb929c8-8fbe-4397-9dbb-9b2b20218538