This one is frequently accessed - Mainly by ::CreateReportBlocks and
is visible in performance profiles (although not very much).
By using webrtc::flat_map, better data cache locality is expected.
Bug: webrtc:12689
Change-Id: Ic2ebcad806788074b2b4cb244a25395a48df1852
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/232541
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35054}