DevTools: migrate first/secondElement to main/sidebarElement in SplitView.

R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/134193010

git-svn-id: svn://svn.chromium.org/blink/trunk@166209 bbb929c8-8fbe-4397-9dbb-9b2b20218538