DevTools: Remove unused SidebarOverlay class.

R=dgozman@chromium.org, dgozman, pfeldman

Review URL: https://codereview.chromium.org/190143004

git-svn-id: svn://svn.chromium.org/blink/trunk@168777 bbb929c8-8fbe-4397-9dbb-9b2b20218538