DevTools: remove View-related methods from SplitView, expose first and second elements only.

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/149493004

git-svn-id: svn://svn.chromium.org/blink/trunk@166199 bbb929c8-8fbe-4397-9dbb-9b2b20218538