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ProtoDatabaseProvider had an unused refptr to a TaskRunner, this CL
removes it and updates some comments.

Bug: 870813
Change-Id: I3980842c4741f2e84f93dcd8070c338b08cf3b28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538996
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644661}