Adds ability to prevent browser connection updates in layout tests.

This is to make the layout tests deterministic and prevent flakes.

BUG=377736

Review URL: https://codereview.chromium.org/302543006

git-svn-id: svn://svn.chromium.org/blink/trunk@174911 bbb929c8-8fbe-4397-9dbb-9b2b20218538