Delete v8::HeapGraphNode::GetHeapValue

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

Review URL: https://codereview.chromium.org/98633009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18404 ce2b1a6d-e550-0410-aec6-3dcde31c8c00