declarative_net_request: base::StringPiece -> std::string_view

Bug: 691162
Change-Id: I8e38321166eb08e1d4754d38aede4ded4be1d03f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5377805


Reviewed-by: Tim <tjudkins@chromium.org>
Auto-Submit: Helmut Januschka <helmut@januschka.com>
Commit-Queue: Helmut Januschka <helmut@januschka.com>
Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277255}