Object<-->id mapping doesn't depend on a particular snapshot, actually same object may appear in several heap snapshots. The API for converting between id and heap object should be provided by HeapProfiler itself. There is already GetObjectId method which I extended with FindObjectById/ClearObjectIds. As the next step I'm going to deprecate and remove HeapGraphNode::GetHeapValue.

BUG=chromium:324769
LOG=N
R=alph@chromium.org, hpayer@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/93843004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18334 ce2b1a6d-e550-0410-aec6-3dcde31c8c00