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At several points in the past we have wanted a Finch-controllable
filter. This change attempts to add a fairly generic one that would have
met all the previous needs.

Change-Id: Iec25677b9bca532c45844933cf7d5d0d95e00764
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612051Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842117}