Bug 1144660 - Make KeepAliveHandler threadsafe refcounted. r=ehsan

This works because KeepAliveHandler only holds nsMainThreadPtrHolder<> which is
itself thread safe.  It has assertions that the PromiseNativeHandler overrides
are only called on the worker and these assertions are satisfied.

--HG--
extra : commitid : Kqgfb87dkFH
extra : rebase_source : 56371d2993c681d99814535a4b5289330763f9c3

dom/workers/ServiceWorkerManager.cpp

@@ -1622,7 +1622,7 @@ class KeepAliveHandler final : public PromiseNativeHandler
   {}
 
 public:
-  NS_DECL_ISUPPORTS
+  NS_DECL_THREADSAFE_ISUPPORTS
 
   explicit KeepAliveHandler(const nsMainThreadPtrHandle<ServiceWorker>& aServiceWorker)
     : mServiceWorker(aServiceWorker)