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

content/common/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/features.gni")
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//ppapi/features/features.gni")
 import("//sandbox/features.gni")
@@ skipping 387 matching lines @@
     "//services/service_manager/public/interfaces",
     "//services/service_manager/runner/common",
     "//services/ui/public/interfaces",
     "//services/video_capture/public/interfaces",
     "//skia",
     "//storage/common",
     "//third_party/boringssl",
     "//third_party/icu",
     "//third_party/libjingle",
     "//third_party/webrtc/base:rtc_base",
-    "//third_party/webrtc/base:rtc_task_runner",
     "//ui/accessibility",
     "//ui/base",
     "//ui/base/ime",
     "//ui/display",
     "//ui/events/blink",
     "//ui/events/ipc",
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gfx/ipc",
     "//ui/gfx/ipc/color",
@@ skipping 208 matching lines @@
     "//url/mojo:url_mojom_origin",
   ]
 
   overridden_deps = [ "//ipc:mojom" ]
   component_deps = [ "//ipc" ]
 
   export_class_attribute = "CONTENT_EXPORT"
   export_define = "CONTENT_IMPLEMENTATION=1"
   export_header = "content/common/content_export.h"
 }