process_proxy: base::StringPiece -> std::string_view

Bug: 691162
Change-Id: I4c377ae4e6f9d07a7db411599208a70ffbc6311a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5321262


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267370}