Use std::string_view instead of base::StringPiece

Based on crrev.com/c/4294483, it sounds like the current guideline is to
use `std::string_view`.

Bug: None
Change-Id: I5e5f6baa6ca45ab0ef79f6021a8fde0434b5f333
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4724955
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Hong Xu <xuhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1176298}

styleguide/c++/c++-features.md

@@ -1719,10 +1719,7 @@ absl::string_view
 *** promo
 Originally banned due to only working with 8-bit characters. Now it is
 unnecessary because, in Chromium, it is the same type as `std::string_view`.
-Use `base::StringPiece` from `base/strings/`, unless interfacing with
-third-party code, in which case prefer to write the type as `std::string_view`.
-Note `base::StringPiece` implicitly converts to and from `std::string_view`, so
-one typically does not need to write the STL name.
+Please use `std::string_view` instead.
 ***
 
 ### Strings Library <sup>[banned]</sup>