An unordered_map typically provides better performance. Instead of a
compare function, we now need a hash function and equality defined on
{Signature<T>}.

R=mstarzinger@chromium.org

Bug: chromium:862123
Change-Id: Iba71030f91949d7453740c884de1d8a4f921c618
Reviewed-on: https://chromium-review.googlesource.com/1131182
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54404}