DevTools: wrap DebuggerAgent.Location with DebuggerModel.Location.

R=sergeyv@chromium.org

Review URL: https://codereview.chromium.org/220903002

git-svn-id: svn://svn.chromium.org/blink/trunk@170579 bbb929c8-8fbe-4397-9dbb-9b2b20218538