Land layer squashing behind a flag

This patch adds appropriate ASSERTs and flag
checks to ensure that the feature does not affect
tip of tree when the flag is disabled.

Tests and bugs will be added in upcoming patches.

BUG=261605

Review URL: https://codereview.chromium.org/88863002

git-svn-id: svn://svn.chromium.org/blink/trunk@163777 bbb929c8-8fbe-4397-9dbb-9b2b20218538