Bug 1236789. Part 2 - fix assertions when |aFlags == NS_DISPATCH_TAIL|. r=bholley

MozReview-Commit-ID: E5a8zP9cMI6

xpcom/threads/nsThreadPool.cpp

@@ -272,7 +272,8 @@ nsThreadPool::Dispatch(already_AddRefed<nsIRunnable>&& aEvent, uint32_t aFlags)
       NS_ProcessNextEvent(thread);
     }
   } else {
-    NS_ASSERTION(aFlags == NS_DISPATCH_NORMAL, "unexpected dispatch flags");
+    NS_ASSERTION(aFlags == NS_DISPATCH_NORMAL ||
+                 aFlags == NS_DISPATCH_TAIL, "unexpected dispatch flags");
     PutEvent(Move(aEvent), aFlags);
   }
   return NS_OK;