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Use flat_map::at() introduced in https://crrev.com/c/1933849 to replace
find()->second, so the code reads easier.

Change-Id: I4f4e04da30e6c7bb7b8f2a2f9a7e582755295b67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1939994
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Auto-Submit: Jiewei Qian  <qjw@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719880}