DevTools: position overlay based on the view position (with potential left/top offsets).

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/141293004

git-svn-id: svn://svn.chromium.org/blink/trunk@165848 bbb929c8-8fbe-4397-9dbb-9b2b20218538