[base] Remove base::StringPiece::as_string() usages

Bug: 1049498
Change-Id: Ia6fc9f76b45e755fbe67171e6c3513c95932c00f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601364


Commit-Queue: Peter Kasting <pkasting@chromium.org>
Owners-Override: Peter Kasting <pkasting@chromium.org>
Owners-Override: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#877499}
NOKEYCHECK=True
GitOrigin-RevId: 846e1896b78c3f649eba31dab067103625d6db02