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This change adds base::fixed_flat_map and base::fixed_flat_set as
specializations of base::flat_map and base::flat_set using a std::array
as the underlying container. These containers have immutable keys
following their construction, and can be used as constexpr lookup tables
in case the keys (and values) are known at compile time and of
appropriate types.

Bug: 682254
Change-Id: I47725824078beeca5100f502041c4977efde5868
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532247Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826866}