[Web Animations] Implement AnimationPlaybackEvent and AnimationPlaybackEventInit

[Web Animations] Implement AnimationPlaybackEvent and AnimationPlaybackEventInit
https://bugs.webkit.org/show_bug.cgi?id=180647
<rdar://problem/35966325>

Reviewed by Dean Jackson.

We implement the AnimationPlaybackEvent interface and AnimationPlaybackEventInit dictionary
as the first step towards dispatching animation "finish" and "cancel" events.

Animation playback events will be dispatched on Animation objects and expose the timeline time
and animation current time at the time they were constructed. Dispatch itself, which will be
the focus of the next patch, is not synchronous, so it's important to track those values
at creation time.

No new tests yet, followup patches will enable event dispatch and existing Web Platform Tests
will show progressions.

* CMakeLists.txt:
* DerivedSources.make:
* Sources.txt:
* WebCore.xcodeproj/project.pbxproj:
* animation/AnimationPlaybackEvent.cpp: Added.
(WebCore::AnimationPlaybackEvent::AnimationPlaybackEvent):
(WebCore::AnimationPlaybackEvent::bindingsCurrentTime const):
(WebCore::AnimationPlaybackEvent::bindingsTimelineTime const):
* animation/AnimationPlaybackEvent.h: Added.
* animation/AnimationPlaybackEvent.idl: Added.
* animation/AnimationPlaybackEventInit.h: Added.
* animation/AnimationPlaybackEventInit.idl: Added.
* bindings/js/WebCoreBuiltinNames.h:
* dom/EventNames.in:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@225745 268f45cc-cd09-0410-ab3c-d52691b4dbfc