Use bundled SDK 10.12 toolchain for Mac and iOS.

.gn

 # Copyright (c) 2015 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/dotfile_settings.gni")
 
@@ skipping 19 matching lines @@
 default_args = {
   # Webrtc does not support component builds because we are not using the
   # template "component" but we rely directly on "rtc_static_library" and
   # "rtc_shared_library". This means that we cannot use the chromium default
   # value for this argument.
   # This also means that the user can override this value using --args or
   # the args.gn file but this setting will be ignored because we don't support
   # component builds.
   is_component_build = false
 
-  mac_sdk_min = "10.11"
+  mac_sdk_min = "10.12"
 
   # WebRTC does not want to switch to C++14 yet.
   use_cxx11 = true
 }