[web-animations-1] Mark ready and finished promise rejections as handled

Fixes w3c#4556.

Author: birtles

web-animations-1/Overview.bs

@@ -889,6 +889,8 @@ The procedure to <dfn>reset an animation's pending tasks</dfn> for
 1.  [=Apply any pending playback rate=] on |animation|.
 1.  <a lt="reject a Promise">Reject</a> <var>animation</var>'s <a>current ready
     promise</a> with a DOMException named "AbortError".
+1.  Set the \[[PromiseIsHandled]] internal slot of |animation|'s
+    <a>current ready promise</a> to true.
 1.  Let <var>animation</var>'s <a>current ready promise</a> be the result of
     <a lt="create a new resolved Promise">creating a new resolved Promise
     object</a> with value <var>animation</var> in the <a>relevant Realm</a> of
@@ -1725,6 +1727,8 @@ as follows:
         <var>animation</var>.
     1.  <a lt="reject a Promise">Reject</a> the <a>current finished promise</a>
         with a DOMException named "AbortError".
+    1.  Set the \[[PromiseIsHandled]] internal slot of
+        the <a>current finished promise</a> to true.
     1.  Let <a>current finished promise</a> be
         <a>a new promise</a> in the <a>relevant Realm</a> of
         <var>animation</var>.