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Most prefix checks take between 0 and 1 ms. This CL improves the
resolution by logging the check in micros.

The CL also removes the SB2.FilterCheck metric which is not really
needed anymore.

Bug: 787092
Change-Id: I18a81d6eb7891f5a4c8c6fd6e7e956dc653d5379
Reviewed-on: https://chromium-review.googlesource.com/960248
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542976}