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

mojo/edk/platform/BUILD.gn

 # 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.
 
 import("../mojo_edk.gni")
 
 mojo_edk_source_set("platform") {
   sources = [
     "message_loop.h",
-    "message_loop_for_io.h",
     "platform_handle.cc",
     "platform_handle.h",
+    "platform_handle_watcher.h",
     "scoped_platform_handle.h",
     "task_runner.h",
   ]
 
   mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
 
   mojo_edk_public_deps = [ "mojo/edk/util" ]
 
   # TODO(vtl): Remove this dependency.
   deps = [
@@ skipping 21 matching lines @@
     "aligned_alloc_unittest.cc",
   ]
 
   deps = [
     ":platform",
     "//testing/gtest",
   ]
 
   mojo_sdk_deps = [ "mojo/public/cpp/system" ]
 }