Bug 1151646 - Spin the event loop one extra time to clean up all CC'd objects before worker shutdown, r=khuey.

dom/workers/RuntimeService.cpp

@@ -2762,15 +2762,18 @@ WorkerThreadPrimaryRunnable::Run()
 #endif
     }
 
-    // Destroy the main context.  This will unroot the main worker global and
-    // GC.  This is not the last JSContext (WorkerJSRuntime maintains an
-    // internal JSContext).
+    // Destroy the main context. This will unroot the main worker global and GC,
+    // which should break all cycles that touch JS.
     JS_DestroyContext(cx);
 
+    // Before shutting down the cycle collector we need to do one more pass
+    // through the event loop to clean up any C++ objects that need deferred
+    // cleanup.
+    mWorkerPrivate->ClearMainEventQueue(WorkerPrivate::WorkerRan);
+
     // Now WorkerJSRuntime goes out of scope and its destructor will shut
-    // down the cycle collector and destroy the final JSContext.  This
-    // breaks any remaining cycles and collects the C++ and JS objects
-    // participating.
+    // down the cycle collector. This breaks any remaining cycles and collects
+    // any remaining C++ objects.
   }
 
   mWorkerPrivate->SetThread(nullptr);

dom/workers/WorkerPrivate.h

@@ -1210,6 +1210,9 @@ public:
     return mCancelAllPendingRunnables;
   }
 
+  void
+  ClearMainEventQueue(WorkerRanOrNot aRanOrNot);
+
   void
   OnProcessNextEvent(uint32_t aRecursionDepth);
 
@@ -1252,9 +1255,6 @@ private:
                 WorkerType aWorkerType, const nsACString& aSharedWorkerName,
                 WorkerLoadInfo& aLoadInfo);
 
-  void
-  ClearMainEventQueue(WorkerRanOrNot aRanOrNot);
-
   bool
   MayContinueRunning()
   {