WebUI: Declare Polymer 2 HTML bundle as output in optimize_webui.gni.

Test:
 - Build chrome once, with optimize_webui=true
 - rm out/<out_folder>/gen/chrome/browser/resources/settings/vulcanized.p2.html.
 - Build chrome again. There should be no errors.

Before this CL an error is thrown, because the file is not re-generated and GRIT
fails to find it. After this CL, the file is correctly re-generated.

Bug: 862472
Change-Id: Idd3896a74b8791558719052f145bb3a500005daf
Reviewed-on: https://chromium-review.googlesource.com/1159276Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580027}

chrome/browser/resources/optimize_webui.gni

@@ -40,6 +40,11 @@ template("optimize_webui") {
     foreach(_out, invoker.html_out_files) {
       outputs += [ "$target_gen_dir/$_out" ]
     }
+    if (defined(invoker.html_out_files_polymer2)) {
+      foreach(_out, invoker.html_out_files_polymer2) {
+        outputs += [ "$target_gen_dir/$_out" ]
+      }
+    }
     foreach(_out, invoker.js_out_files) {
       outputs += [ "$target_gen_dir/$_out" ]
     }