EDK: Remove mojo::platform::MessageLoopForIO and add PlatformHandleWatcher.

mojo/edk/platform/message_loop_for_io.h

-// Copyright 2015 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.
-
-// This file provides an interface for "message loops for I/O", which are used
-// within the EDK itself.
-
-#ifndef MOJO_EDK_PLATFORM_MESSAGE_LOOP_FOR_IO_H_
-#define MOJO_EDK_PLATFORM_MESSAGE_LOOP_FOR_IO_H_
-
-#include "mojo/edk/platform/message_loop.h"
-#include "mojo/public/cpp/system/macros.h"
-
-namespace mojo {
-namespace platform {
-
-// Interface for "message loops for I/O", which are |MessageLoop|s that can
-// watch native handles (file descriptors).
-// TODO(vtl): Currently, we don't add any methods. Obviously, we'll need to do
-// so in the future. (Currently, things will just reach out in appropriately and
-// get the |base::MessageLoopForIO|.)
-class MessageLoopForIO : public MessageLoop {
- public:
-  ~MessageLoopForIO() override {}
-
- protected:
-  MessageLoopForIO() {}
-
- private:
-  MOJO_DISALLOW_COPY_AND_ASSIGN(MessageLoopForIO);
-};
-
-}  // namespace platform
-}  // namespace mojo
-
-#endif  // MOJO_EDK_PLATFORM_MESSAGE_LOOP_FOR_IO_H_