[Brave News]: Fix text rendering on backgrounds with opacity.

[fallaciousreasoning]

Resolves brave/brave-browser#26366

There are two parts to the solution:

1. Don't paint the initial background of the MdTextButton unless we're in kOld mode.
2. SetTextSubpixelRenderingEnabled(false) if we have any opacity.

Before:

After:

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Enable the brave://flags/#brave-news-subscribe-button flag
2. Navigate to https://theatlantic.com/
3. When the Brave News Subscribe button appears in the toolbar, the text should be legible for Follow, Unfollow and Manage Feeds

[simonhong]

Below my local build on win10. It's not as bad as the issue, but it has similar symptoms.
Manage Feeds seems thicker. And this PR doesn't give any difference..

[iefremov]

chromium_src lgtm

[fallaciousreasoning]

@simonhong, I think this is fixed, would you mind taking another look?

[simonhong]

++ 👍🏼

[stephendonner]

Verified PASSED using

Brave	1.47.90 Chromium: 108.0.5359.48 (Official Build) nightly (64-bit)
Revision	18ceeca0d99318e70c00d2e04d88aa55488b5c63-refs/branch-heads/5359@{#854}
OS	Windows 10 Version 22H2 (Build 19045.2311)

Steps:

1. installed 1.47.90
2. launched Brave
3. opened brave://flags
4. set brave://flags/#brave-news-subscribe-button to Enabled
5. clicked Relaunch
6. loaded theatlantic.com
7. clicked on the Manage Subscriptions icon
8. clicked Follow
9. looked at the resulting UI

Confirmed the Follow, Unfollow, and Manage Feeds were clearly legible

Follow	Unfollow