More Windows build fixes, some tidying in mojo_main

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 150cc24cf4f002035e9a5dc4a81a818ca930c481..b85b88d3ba4f0ff78d3c02401fa29d9f6a4fefad 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -7,8 +7,14 @@ group("root") {
 
   deps = [
     "//mojo",
-    "//sky",
   ]
+
+  if (!is_win) {
+    deps += [
+      "//sky",
+    ]
+  }
+
   if (!is_android && !is_win) {
     deps += [ "//tools/xdisplaycheck" ]
   }