Bug 1420178 - Add assertion to ensure the custom element reactions aren't scheduled to BackupQueue if they are from author code; r=smaug

MozReview-Commit-ID: BSna1j1NBSX

--HG--
extra : rebase_source : 0a76832477429c146ade2c597a2e13f83835bc2a

dom/base/CustomElementRegistry.cpp

@@ -1061,6 +1061,8 @@ CustomElementReactionsStack::Enqueue(Element* aElement,
   // Add element to the backup element queue.
   MOZ_ASSERT(mReactionsStack.IsEmpty(),
              "custom element reactions stack should be empty");
+  MOZ_ASSERT(!GetIncumbentGlobal(),
+             "Custom element reaction should not be scheduled to backup queue if it is from author code");
   MOZ_ASSERT(!aReaction->IsUpgradeReaction(),
              "Upgrade reaction should not be scheduled to backup queue");
   mBackupQueue.AppendElement(aElement);