mojo: break struct headers into headers and source

mojo/public/bindings/generators/cpp_struct.template

-// Copyright $year 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.
-
-#ifndef $header_guard
-#define $header_guard
-
-#include "mojo/public/bindings/lib/bindings.h"
-
-namespace $namespace {
-
-#pragma pack(push, 1)
-
-class $classname {
- public:
-  static $classname* New(mojo::Buffer* buf) {
-    return new (buf->Allocate(sizeof($classname))) $classname();
-  }
-
-$setters
-
-$getters
-
- private:
-  friend class mojo::internal::ObjectTraits<$classname>;
-
-  $classname() {
-    header_.num_bytes = sizeof(*this);
-    header_.num_fields = $num_fields;
-  }
-  ~$classname();  // NOT IMPLEMENTED
-
-  mojo::internal::StructHeader _header_;
-$fields
-};
-
-MOJO_COMPILE_ASSERT(sizeof($classname) == $size, bad_sizeof_$classname);
-
-#pragma pack(pop)
-
-}  // namespace $namespace
-
-#endif  // $header_guard