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This CL allows proto db to take a filter to filter out returned entries
Test is also included.
This will facilitate the in-progress download database work.

BUG=842245

Change-Id: I63426527743811ab31560e74a8dc45f922333a00
Reviewed-on: https://chromium-review.googlesource.com/1067149
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561590}