This allows to distinguish DOMWindow objects in browser from each other.

R=vitalyr@chromium.org,sgjesse@chromium.org
BUG=https://bugs.webkit.org/show_bug.cgi?id=61177
TEST=cctest/test-heap-profiler/DocumentURL

Review URL: http://codereview.chromium.org/7082012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8111 ce2b1a6d-e550-0410-aec6-3dcde31c8c00