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

third_party/libjingle/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")
 
 config("jingle_unexported_configs") {
   include_dirs = [
     "../../third_party/webrtc_overrides",
     "../../testing/gtest/include",
@@ skipping 25 matching lines @@
 static_library("libjingle") {
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
   public_deps = [
     ":jingle_deps",
 
     # TODO(kjellander): Start cleaning up this target as soon as
     # https://codereview.chromium.org/2022833002/ is landed. The target should
     # be removed entirely if possible.
+    "//third_party/libjingle_xmpp:rtc_task_runner",
     "//third_party/webrtc/base:rtc_base",
-    "//third_party/webrtc/base:rtc_task_runner",
   ]
   deps = [
     "//third_party/webrtc/p2p:rtc_p2p",
   ]
 
   # From libjingle_common.gypi's conditions list.
   if (is_win) {
     cflags = [ "/wd4005" ]
   }
 
@@ skipping 12 matching lines @@
 
   configs += [
     ":jingle_unexported_configs",
     "//third_party/webrtc:common_config",
   ]
   public_configs = [
     ":jingle_public_configs",
     "//third_party/webrtc:common_inherited_config",
   ]
 }