Add field-initializing constructors to generated mojo structs.

mojo/public/cpp/bindings/lib/native_struct.cc

 // 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.
 
 #include "mojo/public/cpp/bindings/native_struct.h"
 
 #include "mojo/public/cpp/bindings/lib/hash_util.h"
 
 namespace mojo {
 
 // static
 NativeStructPtr NativeStruct::New() {
-  NativeStructPtr rv;
-  internal::StructHelper<NativeStruct>::Initialize(&rv);
-  return rv;
+  return NativeStructPtr(base::in_place);
 }
 
 NativeStruct::NativeStruct() {}
 
 NativeStruct::~NativeStruct() {}
 
 NativeStructPtr NativeStruct::Clone() const {
   NativeStructPtr rv(New());
   rv->data = data;
   return rv;
 }
 
 bool NativeStruct::Equals(const NativeStruct& other) const {
   return data == other.data;
 }
 
 size_t NativeStruct::Hash(size_t seed) const {
   return internal::Hash(seed, data);
 }
 
 }  // namespace mojo