Web Animations API: Support dashes in keyframe properties

This change allows element.animate() keyframes to accept CSS properties
in their "dashed" form as well as camel cased.
Example:
element.animate([
  {backgroundColor: 'red'},
  {'background-color': 'green'},
], 1000);

Relevant specs:
http://w3c.github.io/web-animations/#dictdef-keyframe
http://dev.w3.org/csswg/cssom/#idl-attribute-to-css-property


BUG=417594

Review URL: https://codereview.chromium.org/606563002

git-svn-id: svn://svn.chromium.org/blink/trunk@182658 bbb929c8-8fbe-4397-9dbb-9b2b20218538