Add url_pattern_index to subresource_filter WATCHLIST

Bug: None
Change-Id: I1cee98a5e1c8ff35f0276e57a61a637c97e16b3f
Reviewed-on: https://chromium-review.googlesource.com/549375
Reviewed-by: Pavel Kalinnikov <pkalinnikov@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482488}

WATCHLISTS

@@ -1016,7 +1016,7 @@
       'filepath': '^styleguide/',
     },
     'subresource_filter': {
-      'filepath': 'subresource_filter|SubresourceFilter',
+      'filepath': 'subresource_filter|SubresourceFilter|url_pattern_index',
     },
     'supervised_users': {
       'filepath': 'chrome/browser/.*managed_mode'\