GN: Enable msse2 flag in Mac.

It was disabled for some reason, even though in GYP it's enabled.

BUG=626067
NOTRY=True

Review-Url: https://codereview.webrtc.org/2247293002
Cr-Commit-Position: refs/heads/master@{#13780}

webrtc/modules/desktop_capture/BUILD.gn

@@ -177,7 +177,7 @@ if (use_desktop_capture_differ_sse2) {
     configs += [ "../..:common_config" ]
     public_configs = [ "../..:common_inherited_config" ]
 
-    if (is_posix && !is_mac) {
+    if (is_posix) {
       cflags = [ "-msse2" ]
     }
   }