VisibleSelection::nonBoundaryShadowTreeRootNode should return null when its anchor is a shadow root

      TEST=Automated. Added ua-shadow-select-all-crash.html.
BUG=447906

Review URL: https://codereview.chromium.org/848843002

git-svn-id: svn://svn.chromium.org/blink/trunk@188788 bbb929c8-8fbe-4397-9dbb-9b2b20218538