CV: Add a runtime flag to warn on render access in c-v locked subtrees

This patch adds a warning that will fire on any content-visibity forced
access. Note this flag is not meant to be enabled as stable, just used
as a tool to temporarily check where the pessimizations of
content-visibility may be.

R=chrishtr@chromium.org, pdr@chromium.org

Change-Id: I8abe052ede330b0619466afab1a61e9c5511d1a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3946540
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Vladimir Levin <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1058120}