[Time Zone Monitor] Make OWNERS' expertise clear

In general, I don't have enough expertise in this code to be able to do
meaningful OWNERS reviews for substantive changes to the implementation,
and I redirect such changes to mark@. However, thanks to my refactoring
of the IPC to Mojo I seem to still be the person with the second-most
knowledge of this code on the team :P. This CL adds comments to the
OWNERS file to make my expertise here explicit.

Change-Id: I262d4da3c1abb2ebbc2c2c6b1d0f1425d4cd3674
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617925Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Etienne Bergeron <etienneb@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843081}

services/device/time_zone_monitor/OWNERS

+# For interaction with Mojo and straightforward changes.
 blundell@chromium.org
+
+# For substantive changes to the implementation.
 mark@chromium.org