Add N+3 redundancy to //url/OWNERS

Change-Id: I6b8d9d9e1aa58e4a8e6d9a1067bf24fe4d101751
Reviewed-on: https://chromium-review.googlesource.com/1100735Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567322}

url/OWNERS

+csharrison@chromium.org
+dcheng@chromium.org
 mkwst@chromium.org
+palmer@chromium.org