[MD History] Vulcanize as part of GN build.

chrome/browser/resources/md_downloads/BUILD.gn

 import("../vulcanize.gni")
 
 vulcanize("vulcanize") {
   host = "downloads"
   html_in_file = "downloads.html"
-  html_out_file = "vulcanized.html"
+  html_out_file = "vulcanized.html.unbuilt"
 
   input_type = "FOLDER"
   input = rebase_path(".", root_build_dir)
   js_out_file = "crisper.js"
   deps = []
 }
+
+css_build("build") {
+  input_files = [ "vulcanized.html.unbuilt" ]
+  output_files = [ "vulcanized.html" ]
+  deps = [
+    ":vulcanize",
+  ]
+}