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From observer_list_threadsafe.h:
  // If the observer to be removed is in the list, RemoveObserver MUST
  // be called from the same thread which called AddObserver.

This rule is currently not respected by DomainReliabilityMonitor, which
prevents the addition of a DCHECK.

With this CL, the rule is respected and |last_network_change_time_|
is read/written from the same thread (no data race).

BUG=

Review-Url: https://codereview.chromium.org/2627523003
Cr-Commit-Position: refs/heads/master@{#443283}