Compile rtc_base_objc for Mac.

webrtc/build/objc_common.gypi

+# Copyright 2016 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.
+
+# ObjC target common prefix header.
+
+{
+  'variables': {
+    'objc_prefix_file': './WebRTC-Prefix.pch',
+  },
+  'xcode_settings': {
+    'CLANG_ENABLE_OBJC_ARC': 'YES',
+    'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'YES',
+    'GCC_PREFIX_HEADER': '<(objc_prefix_file)',
+    'GCC_PRECOMPILE_PREFIX_HEADER': 'YES'
+  },
+}