Add a copy of webrtc's TaskRunner abstraction, which is going to be deleted in webrtc.

remoting/client/ios/BUILD.gn

 # Copyright 2016 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("//remoting/build/config/remoting_build.gni")
 
 group("all") {
   testonly = true
 
   deps = [
@@ skipping 28 matching lines @@
     "utility.h",
     "utility.mm",
   ]
 
   if (!is_chrome_branded) {
     sources += [ "host_preferences_persistence_chromium.mm" ]
   }
 
   public_deps = [
     "//third_party/webrtc/base:rtc_base",
-    "//third_party/webrtc/base:rtc_task_runner",
     "//third_party/webrtc/modules/desktop_capture:primitives",
   ]
 
   deps = [
     "//base",
     "//remoting/protocol",
   ]
 
   libs = [
     "CoreGraphics.framework",
@@ skipping 20 matching lines @@
 
   deps = [
     ":ios_core",
     "//testing/gtest",
   ]
 
   configs += [ "//build/config/compiler:enable_arc" ]
 
   data = []
 }