[XML Viewer crash] Store the reference of pretty-print div as tree to be used during the processing

XMLViewer creates a pretty-print div for rendering XML contents. Once the document onLoad is called upon, the processing of the tree takes place. In this if 
the source xml contains the id="tree", it will get conflicted. The resulting XML viewer
may appear blank if the id="tree" is present at the root node. Even worse, if the conflicting
id="tree" appears within any of the elements after the root, it causes the renderer to crash.

The fix is to store the pretty-print div as tree reference to be used during the internal processing.

R=abarth, pfeldman

BUG=284399

Review URL: https://chromiumcodereview.appspot.com/23604028

git-svn-id: svn://svn.chromium.org/blink/trunk@157201 bbb929c8-8fbe-4397-9dbb-9b2b20218538