DevTools: Get rid of WebInspector.{panel, showPanel, openResource}

R=eustas, pfeldman, vsevik

Review URL: https://codereview.chromium.org/195353002

git-svn-id: svn://svn.chromium.org/blink/trunk@169005 bbb929c8-8fbe-4397-9dbb-9b2b20218538