Make PopupWindows into root namespaces

PopupWindows weren't marked as root namespaces. This was
causing problems for focus-related searches since they often
used isRootNamespace to detect when hitting the root of a
tree.

Bug: 38026649
Test: Ran Docs and verified that default cluster is detected
      properly in popup windows.

Change-Id: If8e1db2bb83626d04770a18c0e5ccb88b3793de4