Mojo: Unify the disparate source and header files into 3

mojo/public/bindings/generators/cpp_templates/interface_proxy.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 $HEADER_GUARD
-
-#include "mojo/public/bindings/lib/message.h"
-#include "$HEADER"
-
-namespace $NAMESPACE {
-
-class ${CLASS}Proxy : public $CLASS {
- public:
-  explicit ${CLASS}Proxy(mojo::MessageReceiver* receiver);
-
-$METHODS
-
- private:
-  mojo::MessageReceiver* receiver_;
-};
-
-}  // namespace $NAMESPACE
-
-#endif  // $HEADER_GUARD