DevTools: NetworkPanel: restore stick-to-bottom feature.

This feature was lost in process of migration to ViewportDataGrid.

BUG=408961

Review URL: https://codereview.chromium.org/516913002

git-svn-id: svn://svn.chromium.org/blink/trunk@181326 bbb929c8-8fbe-4397-9dbb-9b2b20218538