HeapProfiler: integrate FindUntrackedObjects into js code.

In some cases we would like to check untracked objects right from the js code.
Otherwise the objects might be collected rigth before check.

BUG=none
R=hpayer@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/27717003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17270 ce2b1a6d-e550-0410-aec6-3dcde31c8c00