Bug 1593766 relax assertion to handle case of no CycleCollectedJSContext created r=baku

This can happen when addModule() has not be called, but the thread is created
just to send a NotifyWorkletFinished message.

Differential Revision: https://phabricator.services.mozilla.com/D51925

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

dom/worklet/WorkletThread.cpp

@@ -373,7 +373,7 @@ void WorkletThread::Terminate() {
 }
 
 void WorkletThread::TerminateInternal() {
-  AssertIsOnWorkletThread();
+  MOZ_ASSERT(!CycleCollectedJSContext::Get() || IsOnWorkletThread());
 
   mExitLoop = true;