MD Extensions: Specify <base href=...> in final bundled HTML file.

With this change, the user is correctly forwarded to chrome://extensions, if an
invalid URL is entered, for example chrome://extensions/foo/bar.html.

Bug: 828174
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I18598c6b3367bcb972efda64bf56cdd1496f2ed7
Reviewed-on: https://chromium-review.googlesource.com/991083Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547587}

chrome/browser/resources/md_extensions/BUILD.gn

@@ -9,6 +9,7 @@ optimize_webui("build") {
   host = "extensions"
   html_in_files = [ "extensions.html" ]
   html_out_files = [ "vulcanized.html" ]
+  insert_in_head = "<base href=\"chrome://extensions\">"
   input = rebase_path("$target_gen_dir/$unpak_folder", root_build_dir)
   js_out_files = [ "crisper.js" ]