bug 1270916 - allow downcasting AccEvent to AccTreeMutationEvent r=davidb

accessible/base/AccEvent.h

@@ -94,6 +94,7 @@ public:
     eGenericEvent,
     eStateChangeEvent,
     eTextChangeEvent,
+    eTreeMutationEvent,
     eMutationEvent,
     eReorderEvent,
     eHideEvent,
@@ -211,6 +212,13 @@ class AccTreeMutationEvent : public AccEvent
 public:
   AccTreeMutationEvent(uint32_t aEventType, Accessible* aTarget) :
     AccEvent(aEventType, aTarget, eAutoDetect, eCoalesceReorder) {}
+
+  // Event
+  static const EventGroup kEventGroup = eTreeMutationEvent;
+  virtual unsigned int GetEventGroups() const override
+  {
+    return AccEvent::GetEventGroups() | (1U << eTreeMutationEvent);
+  }
 };
 
 /**
@@ -232,7 +240,7 @@ public:
   static const EventGroup kEventGroup = eMutationEvent;
   virtual unsigned int GetEventGroups() const override
   {
-    return AccEvent::GetEventGroups() | (1U << eMutationEvent);
+    return AccTreeMutationEvent::GetEventGroups() | (1U << eMutationEvent);
   }
 
   // MutationEvent
@@ -319,7 +327,7 @@ public:
   static const EventGroup kEventGroup = eReorderEvent;
   virtual unsigned int GetEventGroups() const override
   {
-    return AccEvent::GetEventGroups() | (1U << eReorderEvent);
+    return AccTreeMutationEvent::GetEventGroups() | (1U << eReorderEvent);
   }
 };