[Web Animations] The ready promise should initially be resolved
<rdar://problem/41844711>
The reason why we fail to pass both timing-model/animations/current-time.html (webkit.org/b/186505) and timing-model/animations/set-the-animation-start-time.html (webkit.org/b/186506) is because it's initially pending.
Created attachment 344322 [details] Patch
Committed r233519: <https://trac.webkit.org/changeset/233519>