Fix crash when a node being touched moves to a new document

This is a low-risk fix to restore behavior in this edge-case scenario
to what it was in M35.  See bug 394339 for discussion on what the
behavior really should be in this case.

BUG=393822

Review URL: https://codereview.chromium.org/394883003

git-svn-id: svn://svn.chromium.org/blink/trunk@178314 bbb929c8-8fbe-4397-9dbb-9b2b20218538