Bug 1918579 - don't crash when dropping with content analysis and focus changes r=masayuki,dlp-reviewers,handyman

Differential Revision: https://phabricator.services.mozilla.com/D222290

dom/events/EventStateManager.cpp

@@ -1566,7 +1566,8 @@ void EventStateManager::HandleQueryContentEvent(
 
   // If there is an IMEContentObserver, we need to handle QueryContentEvent
   // with it.
-  if (mIMEContentObserver) {
+  // eQueryDropTargetHittest is not really an IME event, though
+  if (mIMEContentObserver && aEvent->mMessage != eQueryDropTargetHittest) {
     RefPtr<IMEContentObserver> contentObserver = mIMEContentObserver;
     contentObserver->HandleQueryContentEvent(aEvent);
     return;

dom/ipc/BrowserChild.cpp

@@ -2086,6 +2086,8 @@ mozilla::ipc::IPCResult BrowserChild::RecvStoreDropTargetAndDelayEndDragSession(
     mDelayedDropPoint = queryEvent.mRefPoint;
     dragSession->StoreDropTargetAndDelayEndDragSession(
         queryEvent.mReply->mDropElement, queryEvent.mReply->mDropFrame);
+  } else {
+    MOZ_ASSERT(false, "Didn't get reply from eQueryDropTargetHittest event!");
   }
   return IPC_OK();
 }