Mojo: Move system embedder API files to mojo/system/embedder.

mojo/system/platform_channel_pair.cc

-// Copyright 2014 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 "mojo/system/platform_channel_pair.h"
-
-#include "base/logging.h"
-
-namespace mojo {
-namespace embedder {
-
-PlatformChannelPair::~PlatformChannelPair() {
-}
-
-ScopedPlatformHandle PlatformChannelPair::PassServerHandle() {
-  return server_handle_.Pass();
-}
-
-ScopedPlatformHandle PlatformChannelPair::PassClientHandle() {
-  return client_handle_.Pass();
-}
-
-}  // namespace embedder
-}  // namespace mojo