Bug 1149906 - Consider play-pending animations to be playing as well r=dbaron

dom/animation/AnimationPlayer.h

@@ -234,8 +234,11 @@ public:
    */
   bool IsPlaying() const
   {
-    return HasInPlaySource() && // Check we are in the active interval
-           PlayState() == AnimationPlayState::Running; // And not paused
+    // We need to have a source animation in its active interval, and
+    // be either running or waiting to run.
+    return HasInPlaySource() &&
+           (PlayState() == AnimationPlayState::Running ||
+            mPendingState == PendingState::PlayPending);
   }
 
   bool IsRelevant() const { return mIsRelevant; }