Mojo C++ bindings: add StringTraits and ArrayTraits for STL types.

mojo/public/interfaces/bindings/tests/struct_with_traits.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.test;
 
+struct StructWithTraitsElement {
+  int32 value;
+};
+
 struct StructWithTraits {
   bool f_bool;
   uint32 f_uint32;
   uint64 f_uint64;
   string f_string;
   string f_string2;
+  array<string> f_array_string;

[Ken Rockot(use gerrit already), 2016/05/13 14:35:51]

nit: I think it would be clearer for readers if these fields were
f_array_of_strings and f_array_of_elements.

extra nit: I think the term "element" could also be slightly confusing as it's
commonly used to refer to generic array contents. What do you think about making
the struct a NestedStructWithTraits and the field below f_array_of_structs?

[yzshen1, 2016/05/13 16:01:17]

Thanks!

I changed to use NestedStructWithTraits, f_string_array and f_struct_array. Does
that sound okay?

+  StructWithTraitsElement f_element;
+  array<StructWithTraitsElement> f_array_element;
 };
 
 interface TraitsTestService {
   PassStructWithTraits(StructWithTraits s) => (StructWithTraits passed);
 };