Fixes issue #12918, hides empty ad container on www.yahoo.com

[ryanml]

Fixes #12918

This is a pretty straightforward fix, the container for the yahoo billboard ad is identifiable by the element Id Billboard-ad. This adds that container to removeEmptyElements.css, as well as adds www.yahoo.com to the sites that removeEmptyElements.css is loaded on.

I did make a slight stylistic change, I broke out the sites inside the matches[] array on to a new line, as it was getting pretty lengthy on one line (it's presumably going to get longer). Let me know if this isn't preferred and I can revert it :)

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

1. Navigate to https://www.yahoo.com/
2. Confirm that the display on the billboard ad (identified by Id: Billboard-ad) is set to none.

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

@jonathansampson can you review this when you get a chance? 😄

[bsclifton]

LGTM! ++

[ryanml]

@jonathansampson @bsclifton awesome thanks guys :)