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This change implements base::flat_tree::contains(). This mirrors C++20,
which introduces contains() to the associative containers in the STL.
See P0458R2 [1] for further details.

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0458r2.html

Bug: 682254
Change-Id: I4af8ce4fadd4647f7021d374bee4316c110c277d
Reviewed-on: https://chromium-review.googlesource.com/c/1288434Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601196}