Add HttpResponseHeaders::EnumerateHeader() overload using string_view.

The old API returned a bool and copied any found headers to a string,
which is ineffecient, and easy to incorrectly use empty strings. The
new overload returns std::optional<std::string_view> to avoid the
copy, and avoid the need for returning a bool and a string.

Also update a couple consumers in net/ to use the new API, though there
are many more in net and elsewhere.

Bug: 369788644
Change-Id: I1414758f6f7c43cf2b30ea44f60da12744281796
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5892460
Commit-Queue: mmenke <mmenke@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1361108}