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This change modifies base::flat_tree, base::flat_set and base::flat_map
to make the underlying container type configurable. This is consistent
with the standardization proposal.

The default container continues to be a std::vector.

Bug: 682254
Change-Id: Ic430e97fa981f21b3f8b20a9e83c153340ea258b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510249Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823293}