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.

layout/forms/nsFileControlFrame.cpp

@@ -88,7 +88,7 @@ MakeAnonButton(nsIDocument* aDoc, const char* labelKey,
                   NS_LITERAL_STRING("button"), false);
 
   // Set the file picking button text depending on the current locale.
-  nsXPIDLString buttonTxt;
+  nsAutoString buttonTxt;
   nsContentUtils::GetLocalizedString(nsContentUtils::eFORMS_PROPERTIES,
                                      labelKey, buttonTxt);