Enable BraveWorkaroundNewWindowFlash 100% on nightly, 25% on beta.

[goodov]

Increase userbase for #1178

[github-actions]

✅ Test Seed Generated Successfully

To apply the test seed:

1. Desktop: Launch the browser with --variations-pr=1182.
   Android: Set the command line to --variations-pr=1182 in debug menu, restart the browser.
   iOS: Set Variations PR to 1182 in Brave Core Switches debug menu, restart the browser.
2. Wait 5-10 seconds to fetch the seed.
3. Restart the browser to apply the seed.
4. Ensure Active Variations section at brave://version starts with the expected seed version (see below).

Seed Details

Parameter	Value
Version	pull/1182@9aff80d26ed2b0fadc19cbbad2c594a1ea8dfc5c
Uploaded	Mon, 19 Aug 2024 06:24:55 GMT
PR commit	32d478a
Base commit	6e47a28
Merge commit	9aff80d
Serial number	b7a71b08e9f60e35f33beaf506025e06

[kjozwiak]

Quick Note: used the same STR/process as per #1178 (comment) when we initially enabled WorkaroundNewWindowFlash on Nightly at 50%.

Nightly verification(s)

Nighty was already verified/checked as per #1178 (comment) when we initially enabled WorkaroundNewWindowFlash on Nightly starting at 50%. There's really no need to re-check Nightly as we're just increasing the % from 50% -> 100%.

BETA verification(s)

Verification PASSED on Win 11 x64 using the following build(s):

Brave | 1.70.85 Chromium: 128.0.6613.40 (Official Build) beta (64-bit)
-- | --
Revision | 212d0e8e26ad5df0c9e2137d8c710b83e721b6b1
OS | Windows 11 Version 23H2 (Build 22631.4037)

WorkaroundNewWindowFlash:Enabled Case - PASSED

• launched 1.70.85 Chromium: 128.0.6613.40 using brave.exe --enable-logging=stderr --variations-pr=1182
• relaunched once again using brave.exe --enable-logging=stderr --variations-pr=1182
• ensured that pull/1182@9aff80d26ed2b0fadc19cbbad2c594a1ea8dfc5c is appearing under brave://version
• ensured that WorkaroundNewWindowFlash:Enabled via brave://version
• ensured that minimizing/maximizing the browser didn't create a white flash when the window is being restored
• ensured that opening Brave doesn't produce a white flash

fixedFlashWindowBeta.mp4

WorkaroundNewWindowFlash:Disabled Case - PASSED

• launched 1.70.85 Chromium: 128.0.6613.40 using brave.exe --enable-logging=stderr --variations-pr=1182
• relaunched once again using brave.exe --enable-logging=stderr --variations-pr=1182
• ensured that pull/1182@9aff80d26ed2b0fadc19cbbad2c594a1ea8dfc5c is appearing under brave://version
• ensured that WorkaroundNewWindowFlash:Disabled via brave://version
• ensured that minimizing/maximizing the browser produced a white flash when the window is being restored
• ensured that opening Brave produced a white flash

reproducedWindowFlash.mp4

[kjozwiak]

Uplift into production approved after deliberating with @brave/uplift-approvers. QA has verified the PR on BETA as per #1182 (comment). Nightly was also verified previously via #1178 (comment).