Add //mojo/edk/platform and move platform_task_runners.h there.

mojo/edk/embedder/BUILD.gn

 # 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.
 
 import("../mojo_edk.gni")
 
 mojo_edk_source_set("embedder") {
   # This isn't really a standalone target; it must be linked into the
   # mojo_system_impl component.
   mojo_edk_visibility = [ "mojo/edk/system" ]
@@ skipping 13 matching lines @@
     # hopefully be dead-stripped.
     "test_embedder.cc",
     "test_embedder.h",
   ]
 
   public_deps = [
     ":delegates",
     ":platform",
   ]
 
-  mojo_sdk_deps = [ "mojo/public/platform/native:system_impl_private_api" ]
+  mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
 
-  mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
+  mojo_edk_public_deps = [ "mojo/edk/platform" ]
 
   deps = [
     "//base",
   ]
 
+  mojo_sdk_deps = [ "mojo/public/platform/native:system_impl_private_api" ]
+
   mojo_edk_deps = [ "mojo/edk/util" ]
 }
 
 mojo_edk_source_set("platform") {
   # This isn't really a standalone target; it must be linked into the
   # mojo_system_impl component.
   visibility = [ ":embedder" ]
 
   mojo_edk_visibility = [ "mojo/edk/system/*" ]
 
   sources = [
     "platform_channel_pair.cc",
     "platform_channel_pair.h",
     "platform_channel_utils.cc",
     "platform_channel_utils.h",
     "platform_handle.cc",
     "platform_handle.h",
     "platform_handle_utils.h",
     "platform_handle_utils_posix.cc",
     "platform_handle_vector.h",
     "platform_shared_buffer.h",
     "platform_support.h",
-    "platform_task_runner.h",
     "scoped_platform_handle.h",
     "simple_platform_shared_buffer.cc",
     "simple_platform_shared_buffer.h",
     "simple_platform_support.cc",
     "simple_platform_support.h",
   ]
 
   mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
 
   deps = [
@@ skipping 38 matching lines @@
   ]
 
   mojo_edk_deps = [
     "mojo/edk/base_edk",
     "mojo/edk/test:test_support",
     "mojo/edk/system",
     "mojo/edk/system/test",
     "mojo/edk/util",
   ]
 }