bug 1329814 - fix race condition when shutting down a tab r=smaug

When TabChild recieves the Destroy message from its parent the doc accessibles
for that tab are shut down.  However if the Shutdown message sent by
DocAccessible::Shutdown() isn't recieved before the child process handles the
runnable that sends __delete__ to the tab parent actor the parent can send a
message to the already shut down doc accessible child actor.

dom/ipc/TabParent.cpp

@@ -359,6 +359,10 @@ TabParent::DestroyInternal()
 
   RemoveWindowListeners();
 
+  if (DocAccessibleParent* tabDoc = GetTopLevelDocAccessible()) {
+    tabDoc->Destroy();
+  }
+
   // If this fails, it's most likely due to a content-process crash,
   // and auto-cleanup will kick in.  Otherwise, the child side will
   // destroy itself and send back __delete__().