Stop observing prefs in MainController on Shutdown

Bug: 893486
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Iba70383fe0993c7c921a118197197497c1d42572
Reviewed-on: https://chromium-review.googlesource.com/c/1270899
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599149}
1 file changed