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.

dom/html/HTMLSelectElement.cpp

@@ -1825,7 +1825,7 @@ HTMLSelectElement::GetValidationMessage(nsAString& aValidationMessage,
 {
   switch (aType) {
     case VALIDITY_STATE_VALUE_MISSING: {
-      nsXPIDLString message;
+      nsAutoString message;
       nsresult rv = nsContentUtils::GetLocalizedString(nsContentUtils::eDOM_PROPERTIES,
                                                        "FormValidationSelectMissing",
                                                        message);