HTML Imports: Let document.defaultView null for imports.

This follows the lates spec update: https://www.w3.org/Bugs/Public/show_bug.cgi?id=23170

R=dglazkov@chromium.org
BUG=408272
TEST=import-defaultView.html, import-script-prototypes.html

Review URL: https://codereview.chromium.org/560183002

git-svn-id: svn://svn.chromium.org/blink/trunk@181826 bbb929c8-8fbe-4397-9dbb-9b2b20218538