bug 1270916 - allow keeping AccTreeMutation in a list r=davidb

Soon we will use the list to track the order of the events.

accessible/base/AccEvent.cpp

@@ -42,7 +42,23 @@ AccEvent::AccEvent(uint32_t aEventType, Accessible* aAccessible,
 ////////////////////////////////////////////////////////////////////////////////
 // AccEvent cycle collection
 
-NS_IMPL_CYCLE_COLLECTION(AccEvent, mAccessible)
+NS_IMPL_CYCLE_COLLECTION_CLASS(AccEvent)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(AccEvent)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mAccessible)
+  if (AccTreeMutationEvent* tmEvent = downcast_accEvent(tmp)) {
+    tmEvent->SetNextEvent(nullptr);
+    tmEvent->SetPrevEvent(nullptr);
+  }
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(AccEvent)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mAccessible)
+  if (AccTreeMutationEvent* tmEvent = downcast_accEvent(tmp)) {
+    CycleCollectionNoteChild(cb, tmEvent->NextEvent(), "mNext");
+    CycleCollectionNoteChild(cb, tmEvent->PrevEvent(), "mPrevEvent");
+  }
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_ROOT_NATIVE(AccEvent, AddRef)
 NS_IMPL_CYCLE_COLLECTION_UNROOT_NATIVE(AccEvent, Release)

accessible/base/AccEvent.h

@@ -219,6 +219,15 @@ public:
   {
     return AccEvent::GetEventGroups() | (1U << eTreeMutationEvent);
   }
+
+  void SetNextEvent(AccTreeMutationEvent* aNext) { mNextEvent = aNext; }
+  void SetPrevEvent(AccTreeMutationEvent* aPrev) { mPrevEvent = aPrev; }
+  AccTreeMutationEvent* NextEvent() const { return mNextEvent; }
+  AccTreeMutationEvent* PrevEvent() const { return mPrevEvent; }
+
+private:
+  RefPtr<AccTreeMutationEvent> mNextEvent;
+  RefPtr<AccTreeMutationEvent> mPrevEvent;
 };
 
 /**