[HTML Imports] Add a test to ensure XHR-ed document doesn't load imports.

TEST=xhr.html
BUG=none
R=dglazkov

Review URL: https://chromiumcodereview.appspot.com/19500018

git-svn-id: svn://svn.chromium.org/blink/trunk@155230 bbb929c8-8fbe-4397-9dbb-9b2b20218538