Update c/b/ui/extensions OWNERS file

Too many per-file rules with the addition of files like
icon_with_badge_image_source.*.  Substitute them for a comment in the file.

BUG=None

Review URL: https://codereview.chromium.org/1645683002

Cr-Commit-Position: refs/heads/master@{#371926}

chrome/browser/ui/extensions/OWNERS

+# App-y stuff
 benwells@chromium.org
 
-per-file extension_action*=finnur@chromium.org
-per-file extension_action*=rdevlin.cronin@chromium.org
-per-file extension_installed_bubble*=finnur@chromium.org
-per-file extension_installed_bubble*=rdevlin.cronin@chromium.org
-per-file extension_message_bubble*=finnur@chromium.org
-per-file extension_message_bubble*=rdevlin.cronin@chromium.org
-per-file extension_toolbar*=finnur@chromium.org
-per-file extension_toolbar*=rdevlin.cronin@chromium.org
+# Extension-y stuff
+finnur@chromium.org
+rdevlin.cronin@chromium.org