crash prompt dialog has both rounded and square corners - follow up to #1841

Description

Crash prompt was added with https://github.com/brave/brave-browser/issues/1841

On macOS Catalina this crash prompt dialog looks odd, it has both rounded and square corners:

Looking at the verification notes under https://github.com/brave/brave-browser/issues/1841#issuecomment-891359676, the prompt looks ok with macOS 12.0, but Windows and Linux behave the same as Catalina having both rounded and squared corners on this prompt.

Steps to Reproduce

Follow test plan for https://github.com/brave/brave-browser/issues/1841 or:

1. Clean profile 1.29.x
2. Open Guest window
3. Go to brave://settings
4. Click on Rewards at the top of the settings page
5. Brave crashes (known issue, logged with https://github.com/brave/brave-browser/issues/15053)
6. Relaunch Brave
7. Get crash report prompt

Actual result:

Light	Dark

Expected result:

Rounded or squared corners on the prompt, not both

Reproduces how often:

Easily

Brave version (brave://version info)

Brave | 1.29.74 Chromium: 93.0.4577.58 (Official Build) (x86_64)
-- | --
Revision | c4410ece044414ea42fa4ba328d08195e818a99c-refs/branch-heads/4577@{#1076}
OS | macOS Version 10.15.7 (Build 19H1323)

Version/Channel Information:

• Can you reproduce this issue with the current release? 1.28.x n/a, but 1.29.x yes
• Can you reproduce this issue with the beta channel? yes
• Can you reproduce this issue with the nightly channel? yes

Other Additional Information:

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

Miscellaneous Information:

cc @brave/legacy_qa @rebron @simonhong

[LaurenWags]

Verified passed on macOS x64 10.15.7 (Catalina) with

Brave | 1.31.12 Chromium: 93.0.4577.58 (Official Build) nightly (x86_64)
-- | --
Revision | c4410ece044414ea42fa4ba328d08195e818a99c-refs/branch-heads/4577@{#1076}
OS | macOS Version 10.15.7 (Build 19H1323)

Verified test plan from https://github.com/brave/brave-core/pull/9894#issue-722053375 Confirmed no rounded corners on the prompt

---

Verified PASSED on

Brave	1.31.18 Chromium: 93.0.4577.63 (Official Build) nightly (64-bit)
Revision	ff5c0da2ec0adeaed5550e6c7e98417dac77d98a-refs/branch-heads/4577@{#1135}
OS	Linux

Verified test plan from https://github.com/brave/brave-core/pull/9894#issue-722053375 Confirmed no rounded corners on the prompt

---

Verified PASSED on

Brave	1.31.31 Chromium: 93.0.4577.63 (Official Build) nightly (64-bit)
Revision	ff5c0da2ec0adeaed5550e6c7e98417dac77d98a-refs/branch-heads/4577@{#1135}
OS	Windows 10 OS Version 2009 (Build 22000.176)

Verified test plan from https://github.com/brave/brave-core/pull/9894#issue-722053375 Confirmed no rounded corners on the prompt

[User198263321]

Would be nice if this could be changed with a flag.