Use string_view in some base::FeatureList functions

These all call functions that take string_view already, so we're not relying on the param being std::string to avoid some conversion.

Change-Id: Ifb57a973f7857afef0a6677200256dbc6ff5a07a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6634071
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/main@{#1471837}