Mojo: Unify the disparate source and header files into 3

mojo/public/bindings/generators/cpp_templates/interface_serialization.h-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 $HEADER_GUARD
-
-#include <string.h>
-
-$INCLUDES
-
-namespace $NAMESPACE {
-namespace internal {
-
-$NAMES
-
-#pragma pack(push, 1)
-
-$PARAM_CLASSES
-#pragma pack(pop)
-
-}  // namespace internal
-}  // namespace $NAMESPACE
-
-namespace mojo {
-namespace internal {
-
-$PARAM_TEMPLATES
-}  // namespace internal
-}  // namespace mojo
-
-#endif  // $HEADER_GUARD