Web Animations: Correctly apply timing functions to CSS animations

Timing functions for CSS animations should be applied per-keyframe, whereas in
the Web Animations model, timing functions apply to the entire animation. To
solve this, this patch introduces a new ChainedTimingFunction, which chains
together several 'primitive' timing functions.

This causes 5 tests to start passing in the web-animations-css virtual suite.

Note that the update to the web-animations-css expected result for
missing-keyframe-properties-timing-function.html is due to the fact that we
don't yet correctly handle keyframes which specify only a subset of the
properties animated by the animation. See the FIXME in
StyleResolver::resolveKeyframes().

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23542026

git-svn-id: svn://svn.chromium.org/blink/trunk@157982 bbb929c8-8fbe-4397-9dbb-9b2b20218538