Bug 1232931 Return null instead of throwing if swm.getWorkerByID() cannot find the worker. r=ochameau IGNORE IDL for comment only change

dom/interfaces/base/nsIServiceWorkerManager.idl

@@ -57,7 +57,8 @@ interface nsIServiceWorkerRegistrationInfo : nsISupports
   // Allows to get the related nsIServiceWorkerInfo for a given
   // nsIWorkerDebugger. Over time we shouldn't need this anymore,
   // and instead always control then nsIWorkerDebugger from
-  // nsIServiceWorkerInfo and not the other way around.
+  // nsIServiceWorkerInfo and not the other way around.  Returns
+  // null if the service worker is no longer registered.
   nsIServiceWorkerInfo getWorkerByID(in unsigned long long aID);
 
   void addListener(in nsIServiceWorkerRegistrationInfoListener listener);

dom/workers/ServiceWorkerManager.cpp

@@ -508,9 +508,7 @@ ServiceWorkerRegistrationInfo::GetWorkerByID(uint64_t aID, nsIServiceWorkerInfo
   MOZ_ASSERT(aResult);
 
   RefPtr<ServiceWorkerInfo> info = GetServiceWorkerInfoById(aID);
-  if (NS_WARN_IF(!info)) {
-    return NS_ERROR_FAILURE;
-  }
+  // It is ok to return null for a missing service worker info.
   info.forget(aResult);
   return NS_OK;
 }