Add an assertion to make sure that event.path used in user shadow trees doesn't leak nodes in user-agent shadow trees.
This assertion should always pass because insertion points in user shadow trees can't select nodes from user-agent shadow trees.

TEST=No behavior change
BUG=None

Review URL: https://codereview.chromium.org/134863008

git-svn-id: svn://svn.chromium.org/blink/trunk@166726 bbb929c8-8fbe-4397-9dbb-9b2b20218538