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This change adds an extract() and replace() API to base::flat_tree which
are inspired by the equivalently named API mentioned in the
standardization proposal of std::flat_map and std::flat_set.

References:
- https://wg21.link/p0429
- https://wg21.link/p1222

Bug: 682254
Change-Id: If517376aa7a682064d55882a0eecf4ff1ce89eef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091352Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747762}