Use standard routine to compute pointer hashs in heap profiler

BUG=None
R=alph@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/22300005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16069 ce2b1a6d-e550-0410-aec6-3dcde31c8c00