EDK: Move {platform_handle,scoped_platform_handle}.* to //mojo/edk/platform.

mojo/edk/embedder/platform_handle.h

-// Copyright 2013 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 MOJO_EDK_EMBEDDER_PLATFORM_HANDLE_H_
-#define MOJO_EDK_EMBEDDER_PLATFORM_HANDLE_H_
-
-namespace mojo {
-namespace embedder {
-
-// A |PlatformHandle| is just a file descriptor on POSIX.
-struct PlatformHandle {
-  PlatformHandle() : fd(-1) {}
-  explicit PlatformHandle(int fd) : fd(fd) {}
-
-  void CloseIfNecessary();
-
-  bool is_valid() const { return fd != -1; }
-
-  int fd;
-};
-
-}  // namespace embedder
-}  // namespace mojo
-
-#endif  // MOJO_EDK_EMBEDDER_PLATFORM_HANDLE_H_