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flat_map can be more efficiently constructed when it only has to sort
once, which can be done by constructing the initial data in a vector
(and ideally, as here, by reserving its capacity).

Change-Id: Iea865434fe17c7b06a21d62565b447332c521cca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605881
Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840420}