DevTools: remove SidebarView, use SplitView instead. Remove default sidebar creation from Panel.

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/132113005

git-svn-id: svn://svn.chromium.org/blink/trunk@166195 bbb929c8-8fbe-4397-9dbb-9b2b20218538