DevTools: make InspectorView.panel and showPanel private. They are going to be async.

R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/609373002

git-svn-id: svn://svn.chromium.org/blink/trunk@182849 bbb929c8-8fbe-4397-9dbb-9b2b20218538