libc++'s _LIBCPP_DEBUG gets upset when comparators take non-const
references.

Bug: chromium:893810
Change-Id: I838ff08bfd53893984f0ce41a9d78d6f1d80a324
Reviewed-on: https://chromium-review.googlesource.com/c/1272184Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56533}