Web Animations CSS: Add to experimental web platform features

This enables use of the Web Animations engine for CSS animations and
transitions when running with experimental web platform features enabled.

It also removes the existing web-animations-css virtual LayoutTests suite
(which ran with --enable-web-animations-css) and replaces it with a new
legacy-animations-engine virtual suite (which runs with
--disable-web-animations-css). Expected results are relocated appropriately.

The change in the expected results for the main animations/ and transitions/
directories document the behavioral progressions introduced by the switch to
the Web Animations engine.

R=dstockwell
BUG=258896, 322272

Review URL: https://codereview.chromium.org/70903004

git-svn-id: svn://svn.chromium.org/blink/trunk@162504 bbb929c8-8fbe-4397-9dbb-9b2b20218538