Stop handling the case where oldDocument and newDocument are the same in adoptDocument()

Stop handling the case where oldDocument and newDocument are the same in
NodeListsNodeData::adoptDocument(). Replace the if check by an assertion as all
the callers already make sure to call this method only if the oldDocument and
the newDocument are different.

R=eseidel, abarth

Review URL: https://codereview.chromium.org/159913004

git-svn-id: svn://svn.chromium.org/blink/trunk@166981 bbb929c8-8fbe-4397-9dbb-9b2b20218538