The stability matters more for maps that it did for sets, so I think it makes sense if all of the flat_set/map containers are stable sorted.

Since we are unlikely to switch out the storage type, I made constructors that take a moved vector. I want to use this pattern to
collect base::Values and then construct a map from it all at once.

Review-Url: https://codereview.chromium.org/2776793002
Cr-Commit-Position: refs/heads/master@{#463094}