brave-browser: Custom adblock rules should allow cosmetic filters

Test plan

See https://github.com/brave/brave-core/pull/4700

Description

Custom adblock rules doesn’t work

Steps to Reproduce

1. Install 0.64.x build
2. Add a custom rule for a site in brave://adblock
3. Visit the page for which the custom rule was added, doesn’t block [Detailed STR on https://github.com/brave/brave-core/pull/3121]

Actual result:

Custom adblock rule doesn’t work. Element is still loaded after adding the rule

Expected result:

Should block elements based on custom rule

Reproduces how often:

Easy

Brave version (brave://version info)

Brave	0.64.72 Chromium: 74.0.3729.131 (Official Build) beta(64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Linux

Version/Channel Information:

• Can you reproduce this issue with the current release? N/A
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the dev channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

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

Miscellaneous Information:

Context menu CSS blocker works fine but isn’t always blocking the right element. This didnt work on muon as well https://github.com/brave/browser-laptop/issues/12308 cc: @emerick