Bug 1494978 - Use LookupOrAdd to simplify hashtable lookup in mouse event coalescence; r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D7179

--HG--
extra : moz-landing-system : lando

dom/ipc/TabChild.cpp

@@ -1618,12 +1618,8 @@ TabChild::RecvRealMouseMoveEvent(const WidgetMouseEvent& aEvent,
                                  const uint64_t& aInputBlockId)
 {
   if (mCoalesceMouseMoveEvents && mCoalescedMouseEventFlusher) {
-    CoalescedMouseData* data = nullptr;
-    mCoalescedMouseData.Get(aEvent.pointerId, &data);
-    if (!data) {
-      data = new CoalescedMouseData();
-      mCoalescedMouseData.Put(aEvent.pointerId, data);
-    }
+    CoalescedMouseData* data = mCoalescedMouseData.LookupOrAdd(aEvent.pointerId);
+    MOZ_ASSERT(data);
     if (data->CanCoalesce(aEvent, aGuid, aInputBlockId)) {
       data->Coalesce(aEvent, aGuid, aInputBlockId);
       mCoalescedMouseEventFlusher->StartObserver();
@@ -1632,7 +1628,6 @@ TabChild::RecvRealMouseMoveEvent(const WidgetMouseEvent& aEvent,
     // Can't coalesce current mousemove event. Put the coalesced mousemove data
     // with the same pointer id to mToBeDispatchedMouseData, coalesce the
     // current one, and process all pending data in mToBeDispatchedMouseData.
-    MOZ_ASSERT(data);
     UniquePtr<CoalescedMouseData> dispatchData =
       MakeUnique<CoalescedMouseData>();