GN: Enable msse2 flag in Mac.

webrtc/modules/desktop_capture/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/ui.gni")
 import("../../build/webrtc.gni")
@@ skipping 159 matching lines @@
   source_set("desktop_capture_differ_sse2") {
     visibility = [ ":*" ]
     sources = [
       "differ_block_sse2.cc",
       "differ_block_sse2.h",
     ]
 
     configs += [ "../..:common_config" ]
     public_configs = [ "../..:common_inherited_config" ]
 
-    if (is_posix && !is_mac) {
+    if (is_posix) {
       cflags = [ "-msse2" ]
     }
   }
 }