Compile the skia library for windows using gn.

gn/BUILDCONFIG.gn

Index: gn/BUILDCONFIG.gn
diff --git a/gn/BUILDCONFIG.gn b/gn/BUILDCONFIG.gn
index 79d06f400b05914371ca9a3947316d9dd4b099ae..e294854aa7c4bc6fe772fd53d90d9b01b52bac27 100644
--- a/gn/BUILDCONFIG.gn
+++ b/gn/BUILDCONFIG.gn
@@ -147,5 +147,10 @@ set_defaults("component") {
   }
 }
 
-# For now, we support GCC-like toolchains, including Clang.
-set_default_toolchain("//gn:gcc_like")
+if (is_win) {
+  # Windows tool chain
+  set_default_toolchain("//gn:msvc")
+} else {
+  # GCC-like toolchains, including Clang.
+  set_default_toolchain("//gn:gcc_like")
+}