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

content/test/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/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
@@ skipping 314 matching lines @@
       "../renderer/media/mock_web_rtc_peer_connection_handler_client.h",
       "../renderer/media/webrtc/mock_peer_connection_dependency_factory.cc",
       "../renderer/media/webrtc/mock_peer_connection_dependency_factory.h",
     ]
 
     deps += [
       "//third_party/libjingle/webrtc:libjingle_webrtc",
       "//third_party/webrtc/api:libjingle_peerconnection",
       "//third_party/webrtc/api:rtc_stats_api",
       "//third_party/webrtc/base:rtc_base",
-      "//third_party/webrtc/base:rtc_task_runner",
       "//third_party/webrtc/media:rtc_media_base",
       "//third_party/webrtc/modules/video_capture",
       "//third_party/webrtc/stats:rtc_stats",
     ]
   }
 
   if (use_glib) {
     configs += [ "//build/config/linux:glib" ]
   }
 
@@ skipping 1268 matching lines @@
       "../renderer/p2p/filtering_network_manager_unittest.cc",
       "../renderer/p2p/ipc_network_manager_unittest.cc",
       "//third_party/webrtc/base/task_queue_unittest.cc",
     ]
     deps += [
       "//third_party/libjingle/webrtc:libjingle_webrtc",
       "//third_party/opus",
       "//third_party/webrtc/api:libjingle_peerconnection",
       "//third_party/webrtc/api:rtc_stats_api",
       "//third_party/webrtc/base:rtc_base",
-      "//third_party/webrtc/base:rtc_task_runner",
       "//third_party/webrtc/media:rtc_media",
       "//third_party/webrtc/modules/desktop_capture:primitives",
       "//third_party/webrtc/modules/video_capture",
       "//third_party/webrtc/stats:rtc_stats_test_utils",
       "//ui/shell_dialogs:shell_dialogs",
     ]
 
     if (is_linux || is_mac || is_win) {
       sources +=
           [ "../browser/media/capture/desktop_capture_device_unittest.cc" ]
@@ skipping 187 matching lines @@
   if (is_android) {
     deps += [ "//testing/android/native_test:native_test_native_code" ]
   }
 }
 
 group("fuzzers") {
   deps = [
     "//content/test/fuzzer",
   ]
 }