DevTools: Follow up fix after r169252

The scrollTop should never go below zero after guardZone is applied.
Otherwise it will always force an update. A similar property
should hold for scrollBottom.

Review URL: https://codereview.chromium.org/200173004

git-svn-id: svn://svn.chromium.org/blink/trunk@169299 bbb929c8-8fbe-4397-9dbb-9b2b20218538