DevTools: Get rid of representedObject usages in ElementsTreeOutline.

In preparation to supporting several occurences of the same node in the DOM tree (e.g. as a distributed nodes)
we want to stop relying on representedObject field that is used as a unique key in treeoutline.js

R=pfeldman

Review URL: https://codereview.chromium.org/702683003

git-svn-id: svn://svn.chromium.org/blink/trunk@184920 bbb929c8-8fbe-4397-9dbb-9b2b20218538