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

content/renderer/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("//build/config/chromecast_build.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//media/media_options.gni")
 import("//ppapi/features/features.gni")
@@ skipping 705 matching lines @@
 
     deps += [
       "//crypto",
       "//third_party/libjingle/webrtc:libjingle_webrtc",
       "//third_party/libvpx",
       "//third_party/opus",
       "//third_party/webrtc/api:libjingle_peerconnection",
       "//third_party/webrtc/api:rtc_stats_api",
       "//third_party/webrtc/api:video_frame_api",
       "//third_party/webrtc/base:rtc_base",
-      "//third_party/webrtc/base:rtc_task_runner",
       "//third_party/webrtc/media:rtc_media",
       "//third_party/webrtc/media:rtc_media_base",
       "//third_party/webrtc/modules/audio_device",
       "//third_party/webrtc/modules/audio_processing",
       "//third_party/webrtc/modules/video_coding:webrtc_h264",
       "//third_party/webrtc/p2p:libstunprober",
       "//third_party/webrtc/p2p:rtc_p2p",
       "//third_party/webrtc/pc:rtc_pc",
       "//third_party/webrtc/stats",
       "//third_party/webrtc/system_wrappers",
@@ skipping 207 matching lines @@
     # For the defines in mojo_media_config.
     public_configs = [ "//media/mojo/services:mojo_media_config" ]
   }
 
   if (!is_component_build) {
     public_deps = [
       ":renderer",
     ]
   }
 }