Requests from about:blank frames are sometimes not collapsed

Environment

• Browser and full version: Chrome 90.0.4430.93
• Extension using this toolkit: Adblock Plus 3.10.2 - Latest 2020-01 build
• Commit hash: N/A

Steps to reproduce

1. Add the filters on the page linked by the testpages issue (#75)

Actual behavior

The image loaded in the about:blank frame is incorrectly allowlisted.

Expected behavior

The image in the about:blank frame to stay blocked, as it does not match the allowlisting filter.

Further information

This has been fixed in v3.

This issue was originally reported as https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/issues/961.