Android: Stop building non-Android WebUI pages unnecessarily.

Test steps:
1) Build Android Chrome once
   ninja -C out/gchrome/ chrome_public_apk
2) Touch a WebUI target that was previously unnecessarily
   included as a dependency, for example:
   touch chrome/browser/resources/md_history/app.js
3) Build Android Chrome again. There should be nothing to do.

Bug: 901261
Change-Id: If81312b54bc785bc4faced24d3f516b029fd1d86
Reviewed-on: https://chromium-review.googlesource.com/c/1324195Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606199}

chrome/browser/BUILD.gn

@@ -5017,7 +5017,7 @@ grit("resources") {
     ]
   }
 
-  if (optimize_webui) {
+  if (!is_android && optimize_webui) {
     deps += [
       "//chrome/browser/resources/md_bookmarks:build",
       "//chrome/browser/resources/md_downloads:build",