[import] Enable document.defaultView for imported documents.

To do that, this exposes internal Document::executingWindow()
instead of Document::domWindow() for defualtView property.

TEST=import-defaultView.html
BUG=334798
R=dglazkov@chromium.org, arv@chromium.org

Review URL: https://codereview.chromium.org/138953013

git-svn-id: svn://svn.chromium.org/blink/trunk@166106 bbb929c8-8fbe-4397-9dbb-9b2b20218538