Remove border radius from brave extension popup windows

[petemill]

Fix brave/brave-browser#1028

This is unfortunate in two ways:

1. It's an ugly patch. ExtensionPopup is both instantiated on its own (macOS) and as a parent class of ExtensionPopupAura. I believe attempting to create a BraveExtensionPopup that both replaces ExtensionPopup and the parent class of ExtensionPopupAura would lead to a larger web of patches, but happy to be shown otherwise.
2. It fixes the issue of the white strips by removing the border radius entirely. The much preferred option is to support the native window radius clipping the webview, but chromium does not yet support this.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

• Ensure that Rewards and Shields popups have no white strip top and bottom (and no border radius except on Linux, which supports the webview clipping)
• Ensure that non-Brave extensions do still have border radius

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[simonhong]

LGTM.
If we want to set radius, I think BubbleBorder's background paint overriding would be an option.

[bbondy]

Tested and works as described, merging since it's a definite improvement over the ugly bar at the top which pushes down the content as it is now. Pls file a follow up issue in 1.x backlog to re-add border radius if you haven't yet, probably with Simon's suggestion.

[bbondy]

master: 9d3018a
0.56.x: 53b069e
0.55.x: 1adce7d

[bsclifton]

Change looks good- tested and it works well 😄