Bug 1432901 - Part 12: Break cycles manually during shutdown r=yulia

Since mozJSComponentLoader is destroyed after the cycle collector, we need to
break cycles manually before the final cycle collection otherwise we get a
memory leak. Unload() is called at an appropriate earlier point in shutdown.

Differential Revision: https://phabricator.services.mozilla.com/D145567

js/loader/ModuleLoaderBase.h

@@ -180,6 +180,9 @@ class ModuleLoaderBase : public nsISupports {
                             nsISerialEventTarget* aEventTarget =
                                 mozilla::GetMainThreadSerialEventTarget());
 
+  // Called to break cycles during shutdown to prevent memory leaks.
+  void Shutdown();
+
   using LoadedScript = JS::loader::LoadedScript;
   using ScriptFetchOptions = JS::loader::ScriptFetchOptions;
   using ScriptLoadRequest = JS::loader::ScriptLoadRequest;