DevTools: Introduce Debugger.StackTrace structure in protocol.json

This will be used to send async traces from backend to front-end.

BUG=272416
R=pfeldman@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/98953004

git-svn-id: svn://svn.chromium.org/blink/trunk@163099 bbb929c8-8fbe-4397-9dbb-9b2b20218538