Experiment with alternative map types.

This experiment replaces uses of std::map with absl::flat_hash_map
in a transparent way. Using the new wrapper uses finch automatically
to evaluate performance differences.

Bug: 433462519

Change-Id: I2046e4d610c66e9d5cb203be37ce4886d062953d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6652483
Commit-Queue: Olivier Li <olivierli@chromium.org>
Auto-Submit: Olivier Li <olivierli@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1513306}
NOKEYCHECK=True
GitOrigin-RevId: ed4f3c0db7f4e4cefd952717bc2015c5324633b7