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This change unifies the various base::make_span(container) utility
functions and simply perfectly forwards the argument to the span
constructor. Two overloads remain, one that deduces the span's
extent automatically and one that let's the caller specify the extent.

In order to do so this change introduces an internal::Extent struct,
which is inspired by std::extent to deduce the right extent from the
passed in argument. This helper is also used to simplify the internal
EnableIfSpanCompatibleArray helper.

Lastly it adds more tests to make sure make_span deduces the right type
and extent.

Bug: 828324

Change-Id: Ib55fc25ab2ad4227daf05d427e0aadf77c671817
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1878367
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712325}