Bug 1371259 part 2. Change union conversions for non-owning unions to pass a MutableHandle through to the underlying conversion code. r=peterv

dom/bindings/Codegen.py

@@ -10239,9 +10239,14 @@ def getUnionTypeTemplateVars(unionType, type, descriptorProvider,
             ctorArgs=ctorArgs,
             jsConversion=jsConversion)
 
+        if ownsMembers:
+            handleType = "JS::Handle<JS::Value>"
+        else:
+            handleType = "JS::MutableHandle<JS::Value>"
+
         setter = ClassMethod("TrySetTo" + name, "bool",
                              [Argument("JSContext*", "cx"),
-                              Argument("JS::Handle<JS::Value>", "value"),
+                              Argument(handleType, "value"),
                               Argument("bool&", "tryNext"),
                               Argument("bool", "passedToJSImpl", default="false")],
                              inline=not ownsMembers,