Use string_view instead of string to avoid string copies.

`.substr()` on string_view doesn't make a copy. It just returns a new
string_view with moved pointers and/or size.

Change-Id: I4511769a4fb4ae50d467e71febebb5ba29569886
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7426116
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Auto-Submit: Jeffrey Yu <yuje@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Jeffrey Yu <yuje@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1568559}