DevTools: Get rid of WebInspector.panels

R=eustas, pfeldman, yurys
BUG=351662

Review URL: https://codereview.chromium.org/268293003

git-svn-id: svn://svn.chromium.org/blink/trunk@173643 bbb929c8-8fbe-4397-9dbb-9b2b20218538