DevTools: Corrupted snapshot makes inspector unresponsive

_buildPostOrderIndex enters infinite loop trying to print retainers
for nodes that have no retainers. That is now fixed.
Also make _buildPostOrderIndex to give out postorder numbers
to orphan nodes, so the snapshot parsing could proceed further.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/18428006

git-svn-id: svn://svn.chromium.org/blink/trunk@154212 bbb929c8-8fbe-4397-9dbb-9b2b20218538