Start documenting the threading model

webrtc/modules/pacing/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../../webrtc.gni")
 
 rtc_static_library("pacing") {
   sources = [
     "alr_detector.cc",
     "alr_detector.h",
     "bitrate_prober.cc",
     "bitrate_prober.h",
     "paced_sender.cc",
     "paced_sender.h",
     "packet_router.cc",
     "packet_router.h",
   ]
 
   if (!build_with_chromium && is_clang) {
     # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
     suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
   }
 
   deps = [
-    "../..:webrtc_common",
+    "../../:webrtc_common",
     "../../base:rtc_base_approved",
     "../../system_wrappers",
     "../rtp_rtcp",
+    "../utility",
   ]
 }
 
 if (rtc_include_tests) {
   rtc_source_set("pacing_unittests") {
     testonly = true
     sources = [
       "alr_detector_unittest.cc",
       "bitrate_prober_unittest.cc",
       "paced_sender_unittest.cc",
@@ skipping 10 matching lines @@
     ]
 
     # TODO(jschuh): bugs.webrtc.org/1348: fix this warning.
     configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
   }
 }