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This change adds a CHECK(str) into BasicStringPiece's constructor from
`const value_type* str`. This prepares the code base for switching to
std::string_view, since std::string_view(nullptr) would invoke undefined
behavior.

Bug: 1049498
Change-Id: Id5d832bc0586dc16a5e1045ba67602e4dc57317d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407712
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806581}