Avoid Member<>'s on stack in core/css

Per crbug.com/40209908 Member<>'s on the stack have extra overhead and
should be avoided. This change converts them to Member<>&'s, which do
not incur this overhead.

No behavioral changes are expected (other than slightly less Oilpan
overhead).

Bug: 439630585
Change-Id: I541ac7d6208b8a137c2e26b8d4197d7069d6c6c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6859402
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1503398}