WebUI: Replace vulcanize with polymer-bundler

chrome/browser/resources/md_downloads/BUILD.gn

 import("../vulcanize.gni")
 
 vulcanized_unbuilt = "vulcanized.unbuilt.html"
 
 vulcanize("vulcanize") {
   deps = []
   host = "downloads"
-  html_in_file = "downloads.html"
-  html_out_file = vulcanized_unbuilt
+  html_in_files = [ "downloads.html" ]
+  html_out_files = [ vulcanized_unbuilt ]
   input = rebase_path(".", root_build_dir)
   insert_in_head = "<base href=chrome://downloads>"
-  js_out_file = "crisper.js"
+  js_out_files = [ "crisper.js" ]
 }
 
 polymer_css_build("build") {
   input_files = [ vulcanized_unbuilt ]
   output_files = [ "vulcanized.html" ]
   deps = [
     ":vulcanize",
   ]
 }