HTML Imports: Let document.open()/close()/write() throw for imports.

This implements https://www.w3.org/Bugs/Public/show_bug.cgi?id=24808.

TEST=import-document-write.html
R=esprehn@chromium.org,dominicc@chromium.org
BUG=363706

Review URL: https://codereview.chromium.org/239423004

git-svn-id: svn://svn.chromium.org/blink/trunk@171917 bbb929c8-8fbe-4397-9dbb-9b2b20218538