Bug 1386600 - Change nsIStringBundle methods to return |AString| instead of |wstring|. r=emk,sr=dbaron.

This removes about 2/3 of the occurrences of nsXPIDLString in the tree. The
places where nsXPIDLStrings are null-checked are replaced with |rv| checks.

The patch also removes a couple of unused declarations from
nsIStringBundle.idl.

Note that nsStringBundle::GetStringFromNameHelper() was merged into
GetStringFromName(), because they both would have had the same signature.

browser/components/shell/nsWindowsShellService.cpp

@@ -614,10 +614,9 @@ nsWindowsShellService::SetDesktopBackground(nsIDOMElement* aElement,
   NS_ENSURE_SUCCESS(rv, rv);
 
   // e.g. "Desktop Background.bmp"
-  nsString fileLeafName;
-  rv = shellBundle->GetStringFromName
-                      ("desktopBackgroundLeafNameWin",
-                       getter_Copies(fileLeafName));
+  nsAutoString fileLeafName;
+  rv = shellBundle->GetStringFromName("desktopBackgroundLeafNameWin",
+                                      fileLeafName);
   NS_ENSURE_SUCCESS(rv, rv);
 
   // get the profile root directory