brave-browser: Fix cosmetic filters not being applied inside of child frames

Description

Some cosmetics aren’t being applied

Steps to Reproduce

1. Enable Fanboy Annoyances in brave://adblock
2. Open https://instapundit.com/513971/#respond
3. Expect the social elements to be filtered.

Actual result:

Expected result:

Social cosmetics should be removed/filtered out.

Reproduces how often:

Easily, when Annoyances/Social list is enabled.

Brave version (brave://version info)

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

cc: @antonok-edm

Related thread: https://community.brave.com/t/block-something-using-browser-instead-of-ublock-origin/368569/