Hash iterators: Check for concurrent modification and fix 1 place where it happened

Source/core/animation/DocumentTimeline.h

Index: Source/core/animation/DocumentTimeline.h
diff --git a/Source/core/animation/DocumentTimeline.h b/Source/core/animation/DocumentTimeline.h
index 200568385440ed73b6229bb3e8dce711e0272ba2..0a998b3d5e31fe76898db988a732e7e5ea4dfeb7 100644
--- a/Source/core/animation/DocumentTimeline.h
+++ b/Source/core/animation/DocumentTimeline.h
@@ -97,8 +97,10 @@ private:
     double m_zeroTime;
     Document* m_document;
     // AnimationPlayers which will be updated on the next frame
-    // i.e. current, in effect, or had timing changed
-    HashSet<RefPtr<AnimationPlayer> > m_playersNeedingUpdate;
+    // i.e. current, in effect, or had timing changed. This needs
+    // to be a collection that can be updated while we iterate
+    // over it (this happens during pauseAnimationsForTesting).
+    ListHashSet<RefPtr<AnimationPlayer> > m_playersNeedingUpdate;
     HashSet<AnimationPlayer*> m_players;
     bool m_hasOutdatedAnimationPlayer;