Prevent a TreeScope object from being deleted if they hold a reference to the parent in TreeShared.

BUG=260138

Review URL: https://chromiumcodereview.appspot.com/22528002

git-svn-id: svn://svn.chromium.org/blink/trunk@155668 bbb929c8-8fbe-4397-9dbb-9b2b20218538