[Web Animations] Expose promises on Animation interface

[Web Animations] Expose promises on Animation interface
https://bugs.webkit.org/show_bug.cgi?id=180710
<rdar://problem/36000604>

Reviewed by Dean Jackson.

Source/WebCore:

Expose the "ready" and "finished" promises on Animation. A future patch will fulfil or reject them.

* animation/WebAnimation.cpp:
(WebCore::WebAnimation::WebAnimation):
(WebCore::WebAnimation::readyPromiseResolve):
(WebCore::WebAnimation::finishedPromiseResolve):
* animation/WebAnimation.h:
* animation/WebAnimation.idl:

LayoutTests:

Rebase some WPT expectations with progressions due to exposing the "ready" and "finished" promises.

* http/wpt/web-animations/interfaces/Animation/idlharness-expected.txt:
* http/wpt/web-animations/timing-model/animations/current-time-expected.txt:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@225812 268f45cc-cd09-0410-ab3c-d52691b4dbfc