Convert users of EnumerationBase::AsString() to ...::AsStringView()

Avoid minting a new String in the cases where just using the existing
(C-)string is possible.

This converts users of AsString() to AsStringView(), and transforms from
using string operators to using StrCat() for concatenation.

Bug: 430171134
Change-Id: Icfd1b674ccd8073d399fd5ca7bf726ba88ea62a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6912789
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/main@{#1511472}