Web Animations CSS: Snapshot missing property values at 0% and 100% keyframes

The existing code was only creating keyframe snapshots at 0% and 100% if
there were no keyframes at those offsets. This patch ensures snapshots
are created at 0% and 100% offsets per CSS property.

Also adds reuse of AnimatableValues for keyframes with multiple offsets
and re-adds stable_sort().

Fixes crash in virtual test:
virtual/web-animations-css/animations/missing-keyframe-properties-repeating.html

Review URL: https://chromiumcodereview.appspot.com/22336010

git-svn-id: svn://svn.chromium.org/blink/trunk@155883 bbb929c8-8fbe-4397-9dbb-9b2b20218538