DevTools: [Console] prerender more elements in viewport.

This patch figures out the amount of elements to be rendered in viewport
according to the minimum ever-rendered element height. This accounts
for the case when rendered object gets collapsed without
causing a viewport refresh, e.g. tree element gets collapsed.

BUG=390169
R=vsevik, aandrey, dgozman, pfeldman

Review URL: https://codereview.chromium.org/380943006

git-svn-id: svn://svn.chromium.org/blink/trunk@178003 bbb929c8-8fbe-4397-9dbb-9b2b20218538