Heap snapshot: Update user roots definition.

Do not define an object as a user root if its context is not
present in Global handles.

R=mstarzinger@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/17881005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15381 ce2b1a6d-e550-0410-aec6-3dcde31c8c00