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.

parser/html/nsHtml5TreeOperation.cpp

@@ -932,7 +932,7 @@ nsHtml5TreeOperation::Perform(nsHtml5TreeOpExecutor* aBuilder,
       }
 
       nsresult rv;
-      nsXPIDLString message;
+      nsAutoString message;
       if (otherAtom) {
         const char16_t* params[] = { atom->GetUTF16String(),
                                       otherAtom->GetUTF16String() };