Mojo: Unify the disparate source and header files into 3

mojo/public/bindings/generators/cpp_templates/struct_serialization.cc-template

-// Copyright $YEAR The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "$SERIALIZATION_HEADER"
-
-#include <string.h>
-
-$INCLUDES
-
-namespace mojo {
-namespace internal {
-
-// static
-size_t ObjectTraits<$CLASS>::ComputeSizeOf(
-    const $CLASS* $NAME) {
-$SIZES
-}
-
-// static
-$CLASS* ObjectTraits<$CLASS>::Clone(
-    const $CLASS* $NAME, Buffer* buf) {
-  $CLASS* clone = $CLASS::New(buf);
-  memcpy(clone, $NAME, sizeof(*$NAME));
-
-$CLONES
-
-  return clone;
-}
-
-$SERIALIZATION
-
-}  // namespace internal
-}  // namespace mojo