Shields shouldn't be available from internal/chrome:// pages

[stephendonner]

Description

Shields shouldn't be available from internal/chrome:// pages

Steps to Reproduce

1. install 1.38.32
2. launch Brave
3. load brave://flags
4. set Enable Brave Shields v2 to Enabled
5. click to Relaunch Brave
6. in the same internal brave://flags tab, click on the new Shields icon in the URL bar

Actual result:

You get the Shields doorhanger popup, functional

Expected result:

Nothing should happen when you click on the Shields icon on internal chrome:// schemes / pages

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.38.32 Chromium: 99.0.4844.51 (Official Build) nightly (x86_64)
Revision	d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS	macOS Version 11.6.3 (Build 20G415)

Version/Channel Information:

• Can you reproduce this issue with the current release? no
• Can you reproduce this issue with the beta channel? no
• Can you reproduce this issue with the nightly channel? yes

cc @rebron @nullhook @ShivanKaul @pes10k

[stephendonner]

Verified PASSED using

Brave	1.38.60 Chromium: 100.0.4896.46 (Official Build) nightly (x86_64)
Revision	5ca33821b2211805855c77d334353d27c616a7ca-refs/branch-heads/4896@{#584}
OS	macOS Version 11.6.3 (Build 20G415)

Confirmed Shields were disabled on various internal brave:// pages
Confirmed Shields were available by default and blocked content on CNN.com

brave://about	brave://rewards	brave://flags	brave://version