about:blank allowlisting test page

Test page to help verify that the allowlisting of about:blank initiated requests works correctly. The idea is that the image should be generally blocked, but allowed inside the both the iframes as the frame URLs have the ?allow suffix. Install the test Chrome extension, and then check that only the main-frame image is blocked. See https://crbug.com/1201597.

Image Description
Failed ✕ main-frame
HTML iframe
about:blank iframe