Compile the skia library for windows using gn.

third_party/third_party.gni

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 template("third_party") {
   config(target_name + "_public") {
     include_dirs = invoker.public_include_dirs
   }
   source_set(target_name) {
     forward_variables_from(invoker, "*", [ "public_include_dirs" ])
     public_configs = [ ":" + target_name + "_public" ]
 
     # Warnings are just noise if we're not maintaining the code.
-    cflags = [ "-w" ]
+    if (is_win) {
+      cflags = [ "/w" ]
+    } else {
+      cflags = [ "-w" ]
+    }
   }
 }
 
 set_defaults("third_party") {
   configs = default_configs
 }