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This CL does a few things:
1. Replaces HashPrefixMatches which was implemented with a custom binary
   search with a custom iterator and std::binary_search.
2. Adds a perftest stress testing this method.

Running the perftest before and after the binary_search change shows that
using binary_search improves performance by 30-40%. This was measured
on Linux using a release (non-official) build.

It is also imo a code health win, as an iterator is easier to reason
about than a custom binsearch algorithm.

Bug: 787092
Change-Id: Ic8be26b11ef750c70ee241dfb2718a3c0855fc8c
Reviewed-on: https://chromium-review.googlesource.com/959623
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543391}