[iOS] Link with base/maccocoathreadhelper.mm on iOS.

webrtc/base/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("//build/config/crypto.gni")
 import("//build/config/ui.gni")
@@ skipping 503 matching lines @@
     libs += [
       "log",
       "GLESv2",
     ]
   }
 
   if (is_ios) {
     all_dependent_configs = [ ":ios_config" ]
 
     sources += [
+      "maccocoathreadhelper.h",
+      "maccocoathreadhelper.mm",
       "macconversion.cc",
       "macconversion.h",
     ]
   }
 
   if (use_x11) {
     sources += [
       "x11windowpicker.cc",
       "x11windowpicker.h",
     ]
@@ skipping 119 matching lines @@
       "objc/RTCUIApplication.mm",
     ]
   }
 }
 
 source_set("gtest_prod") {
   sources = [
     "gtest_prod_util.h",
   ]
 }