Add struct traits for base::Value.

mojo/common/values.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module mojo.common.mojom;
 
+union Value {
+  NullValue? null_value;

[dcheng, 2016/12/21 02:23:04]

Btw, why does this need to be nullable?

[Sam McNally, 2016/12/21 02:32:53]

It's so the traits can return a null NullValuePtr instead of creating a real
NullValue. The receiver ignores it so a null value is fine.

+  bool bool_value;
+  int32 int_value;
+  double double_value;
+  string string_value;
+  array<uint8> binary_value;
+  DictionaryValue dictionary_value;
+  ListValue list_value;
+};
+
+struct ListValue {
+  array<Value> values;
+};
+
+struct DictionaryValue {
+  map<string, Value> values;
+};
+
+// An empty struct representing a null base::Value.
+struct NullValue {
+};
+
+// To avoid versioning problems for arc. TODO(sammc): Remove ASAP.
 [Native]
-struct ListValue;
-
-[Native]
-struct DictionaryValue;
+struct LegacyListValue;