Mojo: Move platform handle/channel stuff in system to embedder namespace.

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 system {
+namespace embedder {
 
 PlatformChannelPair::~PlatformChannelPair() {
 }
 
 ScopedPlatformHandle PlatformChannelPair::PassServerHandle() {
   return server_handle_.Pass();
 }
 
 ScopedPlatformHandle PlatformChannelPair::PassClientHandle() {
   return client_handle_.Pass();
 }
 
-}  // namespace system
+}  // namespace embedder
 }  // namespace mojo