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Adds some comments clarifying why BrowserStatePolicyConnector isn't a
keyed service. Comments are added in the impl class, the mock class, and
the parent object (both the mock and the real application context).

Change-Id: I3a48a3ac903a3b37c916275ae18b78e77edf691f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2499961Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#821306}