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 e6a8b695ffe29f5a363baed6330ead29674b0646..bdcd59af86eb3bb158fbd0fed76536e9e0d91ce7 100644
--- a/gn/BUILDCONFIG.gn
+++ b/gn/BUILDCONFIG.gn
@@ -139,5 +139,10 @@ 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")
+}