Web Animations: Define easing for each keyframe

The timing function 'easing' is used by sample() to interpolate between keyframes.

See Section 6.16 The Keyframe dictionary
in a draft of the Web Animations spec:
https://dvcs.w3.org/hg/FXTF/raw-file/rework-animation-model/web-animations/index.html#the-keyframe-dictionary

We plan to use easing on keyframes to retire ChainedTimingFunction.


BUG=232278
R=shanestephens,dstockwell,timloh

Review URL: https://codereview.chromium.org/142523005

git-svn-id: svn://svn.chromium.org/blink/trunk@166803 bbb929c8-8fbe-4397-9dbb-9b2b20218538