Mojo: Unify the disparate source and header files into 3

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

-// Copyright $YEAR The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "$HEADER"
-
-#include <stdlib.h>
-
-#include "mojo/public/bindings/lib/message.h"
-#include "mojo/public/bindings/lib/message_builder.h"
-#include "$SERIALIZATION_HEADER"
-
-namespace $NAMESPACE {
-
-${CLASS}Proxy::${CLASS}Proxy(mojo::MessageReceiver* receiver)
-    : receiver_(receiver) {
-}
-
-$IMPLEMENTATIONS
-
-}  // namespace $NAMESPACE