Custom filters sometimes persist after they have been removed

[kzar]

Environment

Adblock Plus built using the next branch of the adblockpluscore ( ​699451fe09c5). (See the code review of #7054 to see the required changes to adblockpluschrome.)

How to reproduce

1. Navigate to ​​https://testpages.adblockplus.org/en/filters/blocking
2. Add the very first suggested filter ||testpages.adblockplus.org/testcasefiles/blocking/addresscomplete/image.jpg via the options page in the advanced tab
3. Refresh page (image is blocked)
4. Open the options page and navigate to the user-defined filters and delete the test filter
5. Refresh the page (image is still blocked)
6. Open the Adblock Plus DevTools panel (the image filter still shows as an unnamed subscription)

Observed behaviour

Image is still blocked after the filter is removed and the page is refreshed.
Our developer tools pane still lists the image filter, but with an unamed subscription.

Expected behaviour

The image is no longer blocked after the filter is removed and the page is refreshed.
The filter is no longer listed.

What to change

We have three options:

1. Make the keyword finding for a filter deterministic. This would make the data structures unbalanced so that some keywords may end up with more than their share of filters.
2. Try all the keyword candidates and use the one that has the filter.
3. Bring back the keyword-by-filter map.

Notes

• The regression was ​caused by this change 30daf36221f1 (#6992).
• The reason the filter is not removed is that the keyword while adding the filter is "addresscomplete" but the keyword while removing the filter is "testcasefiles".
• ​Jon spotted this regression initially, then Manish wrote up the details in a comment which I based this issue on.

[abpbot]

A commit referencing this issue has landed:
​Issue 7181 - Keep a keyword-by-filter map after all

[abpbot]

A commit referencing this issue has landed:
​Issue 7181 - Keep a keyword-by-filter map after all