DevTools: ViewportDataGrid: avoid rendering with incorrect scrollTop.

Fix scrolltop before rendering.

BUG=409766

Review URL: https://codereview.chromium.org/529923002

git-svn-id: svn://svn.chromium.org/blink/trunk@181239 bbb929c8-8fbe-4397-9dbb-9b2b20218538