Build error: chrome://extensions/brave_item_list_more_items.html

[petemill]

Occassionaly the build will fail with the following error:

html_in_files extensions.html --html_out_files vulcanized.html --html_out_files_polymer2 vulcanized.p2.html --js_out_files crisper.js --insert_in_head "<base href=\"chrome://extensions\">"
18:22:30 Traceback (most recent call last):
18:22:30   File "../../chrome/browser/resources/optimize_webui.py", line 271, in <module>
18:22:30     main(sys.argv[1:])
18:22:30   File "../../chrome/browser/resources/optimize_webui.py", line 265, in main
18:22:30     '\n'.join(manifest['_missing']))
18:22:30 Exception: polymer-bundler could not find files for the following URLs:
18:22:30 chrome://extensions/brave_item_list_more_items.html

This was reported by @mihaiplesa on the Windows ia32 platform, but from looking at the cause, it could happen on any platform and depends on build timing and build target re-builds.

There is a missing dependency from chromium's extension webui resources build to brave's extra extension webui resources build.
This has mostly gone unnoticed since we chained the brave extension resources build on to the brave settings webui resources build. Therefore it would work if settings webui was built before extensions, but there is nothing enforcing that order.

The extra resources were added in at brave/brave-core#1615 which is merged up to 0.63.x cc @mkarolin