DevTools: MQs should start at zero in the grid

The left hand icon bar now is drawn through to the bottom of the toolbar, so media queries start at actual zero of the rulers. The vertical ruler had to be enlarged to accommodate.

BUG=411247

Review URL: https://codereview.chromium.org/544013003

git-svn-id: svn://svn.chromium.org/blink/trunk@181743 bbb929c8-8fbe-4397-9dbb-9b2b20218538