Fix invalid toElement call in Document::importNode

The bad cast was added in http://src.chromium.org/viewvc/blink?revision=164574&view=revision

Also made a better error message for ShadowRoot, since its nodeValue is "#document-fragment"

R=inferno@chromium.org
BUG=270033,332677

Review URL: https://codereview.chromium.org/132533002

git-svn-id: svn://svn.chromium.org/blink/trunk@164826 bbb929c8-8fbe-4397-9dbb-9b2b20218538