Replace `std::unordered_map` with `absl::flat_hash_map`.

The Chromium containers guide
(https://chromium.googlesource.com/chromium/src/+/HEAD/base/containers/README.md)
recommends using `absl::flat_hash_map` as a default map choice instead
of `std::unordered_map` when pointer stability isn't needed, and
`absl::node_hash_map` when it is. Both will still generally perform
better and require less overhead than `std::unordered_map`.

Compile-Size: Size increase is unavoidable.
Bug: 473916362
Change-Id: Id8094b2b090de9c4e5a51303526f4176e24bc6e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7552923
Reviewed-by: Reilly Grant <[email protected]>
Auto-Submit: Jeffrey Yu <[email protected]>
Reviewed-by: David Schinazi <[email protected]>
Commit-Queue: Jeffrey Yu <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1584777}
NOKEYCHECK=True
GitOrigin-RevId: ab1dadc8f92615316237914c3279e0e6bde77c10
4 files changed