Fixed space leak in DebuggerAgentSession.

Ownership in this part is still very convoluted and should probably be
cleaned up, this is only the minimal CL needed to fix the leak.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/153623009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19148 ce2b1a6d-e550-0410-aec6-3dcde31c8c00