In cases where it seemed like there might be a high query volume or I had no way of knowing, it is replaced with a std::unordered_map (which base::hash_map is a typedef for already).

In cases where I understood the workload and it was appropriate, base::flat_map is used. In a few places that were create-once-then-read-only, the flat_map optimized construction from a vector is used.

In all other cases, std::map is used.

Review-Url: https://codereview.chromium.org/2829163004
Cr-Commit-Position: refs/heads/master@{#483431}