Cosmetic filter - use !important for display property

[rulatir]

Test plan

See brave/brave-core#3121

Description

The cosmetic filters don't work with elements that set the display property using the style attribute.

Proposed solution: in the rule created by the cosmetic filter, the display property should be !important.

Please implement!

[Detailed STR on PR]

[srirambv]

Probably covered under #904

[rulatir]

Please give it a minute of your time to actually look into the issue. The element in question has an id attribute, so the style rule injected by the cosmetic filter is super simple:

#element-id {
   display: none;
}

But the element has style attribute with display: block, so this overrides display: none in the injected stylesheet. It's pure CSS as CSS behaves. Nothing to analyse further. This has nothing to do with wrong element being matched. Replacing the unique-selector library will not fix this. No tweaks to the selector of the cosmetic filter's rule will fix this. The element being matched is the right one, it's just that the cosmetic filter's style rule is too weak.

[btlechowski]

Verification passed on

Brave	0.69.129 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#3121
For reddit portion I had to lower shields first to show promoted posts.

Verification passed on

Brave	0.69.129 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified the test plan from fix cosmetic filtering brave-core#3121

Verified passed with

Brave	0.69.129 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	macOS Version 10.13.6 (Build 17G5019)

• Verified the test plan from fix cosmetic filtering brave-core#3121