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This change adds missing APIs from https://wg21.link/string.view.ops and
https://wg21.link/string.view.find to base::StringPiece. In particular:

* Use traits_type::copy for copy, add CHECK for OOB
* Add missing default argument to substr()
* Add missing compare() overloads
* Add missing find() overloads
* Add missing rfind() overloads
* Add missing find_first_of() overloads
* Add missing find_last_of() overloads
* Add missing find_first_not_of() overloads
* Add missing find_last_not_of() overloads
* Take StringPieces by value instead of const-ref in these APIs

Bug: 1049498
Change-Id: I6392dd9e713265fda25721d6f2d1a8f3db133b76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632597Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844495}