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

third_party/libjingle_xmpp/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.
 
 # TODO(kjellander): Remove remaining dependencies on the WebRTC codebase.
 import("../webrtc/webrtc.gni")
 
 group("libjingle_xmpp") {
   public_deps = [
     ":rtc_xmllite",
     ":rtc_xmpp",
   ]
 }
 
+rtc_static_library("rtc_task_runner") {
+  sources = [
+    "task_runner/task.cc",
+    "task_runner/task.h",
+    "task_runner/taskparent.cc",
+    "task_runner/taskparent.h",
+    "task_runner/taskrunner.cc",
+    "task_runner/taskrunner.h",
+  ]
+}
+
 rtc_static_library("rtc_xmllite") {
   sources = [
     "xmllite/qname.cc",
     "xmllite/qname.h",
     "xmllite/xmlbuilder.cc",
     "xmllite/xmlbuilder.h",
     "xmllite/xmlconstants.cc",
     "xmllite/xmlconstants.h",
     "xmllite/xmlelement.cc",
     "xmllite/xmlelement.h",
     "xmllite/xmlnsstack.cc",
     "xmllite/xmlnsstack.h",
     "xmllite/xmlparser.cc",
     "xmllite/xmlparser.h",
     "xmllite/xmlprinter.cc",
     "xmllite/xmlprinter.h",
   ]
 
   deps = [
     "//third_party/webrtc/base:rtc_base",
-    "//third_party/webrtc/base:rtc_task_runner",
   ]
   public_deps = [
     "//third_party/expat",
   ]
 }
 
 config("rtc_xmpp_warnings_config") {
   # GN orders flags on a target before flags from configs. The default config
   # adds these flags so to cancel them out they need to come from a config and
   # cannot be on the target directly.
@@ skipping 30 matching lines @@
     "xmpp/xmppstanzaparser.h",
     "xmpp/xmpptask.cc",
     "xmpp/xmpptask.h",
   ]
 
   defines = []
 
   deps = [
     ":rtc_xmllite",
     "//third_party/webrtc/base:rtc_base",
-    "//third_party/webrtc/base:rtc_task_runner",
   ]
   public_deps = [
+    ":rtc_task_runner",
     "//third_party/expat",
   ]
   configs += [ ":rtc_xmpp_warnings_config" ]
 
   if (is_nacl) {
     deps += [ "//native_client_sdk/src/libraries/nacl_io" ]
   }
 
   if (is_posix && is_debug) {
     # The Chromium configs defines this for all posix _except_ for ios & mac.
@@ skipping 14 matching lines @@
       "-Wno-unused-const-variable",
     ]
   }
 }
 
 rtc_test("libjingle_xmpp_unittests") {
   configs += [ ":libjingle_xmpp_unittests_config" ]
 
   deps = [
     ":libjingle_xmpp",
+    ":rtc_task_runner",
 
     # TODO(kjellander): Refactor/remove this dependency. It is needed by
     # third_party/webrtc_overrides/webrtc/base/win32socketinit.cc.
     "//net",
     "//testing/gtest",
   ]
 
   sources = [
     "run_all_unittests.cc",
+    "task_runner/task_unittest.cc",
     "xmllite/qname_unittest.cc",
     "xmllite/xmlbuilder_unittest.cc",
     "xmllite/xmlelement_unittest.cc",
     "xmllite/xmlnsstack_unittest.cc",
     "xmllite/xmlparser_unittest.cc",
     "xmllite/xmlprinter_unittest.cc",
     "xmpp/fakexmppclient.h",
     "xmpp/jid_unittest.cc",
     "xmpp/util_unittest.cc",
     "xmpp/util_unittest.h",
     "xmpp/xmppengine_unittest.cc",
     "xmpp/xmpplogintask_unittest.cc",
     "xmpp/xmppstanzaparser_unittest.cc",
   ]
 }