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This change adds copy and move constructors from the underlying_storage
to base::flat_tree, replacing the previous by value constructor. This
results in a small win for perf and binary size, as the number of
temporaries is reduced.

Bug: 682254
Change-Id: I0c3c8fd6e785556ada363cf406d7f58f22a60284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037674Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738627}