use correct return type

widget/src/mac/nsAppShell.cpp

@@ -86,7 +86,7 @@ NS_IMETHODIMP nsAppShell::Create(int* argc, char ** argv)
 // Enter a message handler loop
 //
 //-------------------------------------------------------------------------
-nsresult nsAppShell::Run()
+NS_IMETHODIMP nsAppShell::Run()
 {
 	if (!mMacPump.get())
 		return NS_ERROR_NOT_INITIALIZED;

widget/src/mac/nsAppShell.h

@@ -60,7 +60,7 @@ class nsAppShell : public nsIAppShell
     // nsIAppShellInterface
   
     NS_IMETHOD              Create(int* argc, char ** argv);
-    virtual nsresult        Run();
+    NS_IMETHOD              Run();
     NS_IMETHOD              Spinup();
     NS_IMETHOD              Spindown();
     NS_IMETHOD              PushThreadEventQueue();