Enable BraveWorkaroundNewWindowFlash 25% Release

[goodov]

1.70.x is now Release, so we can continue rolling this feature. It's already enabled by default in Nightly and Beta.

Related #1178

[github-actions]

✅ Test Seed Generated Successfully

To apply the test seed:

1. Desktop: Launch the browser with --variations-pr=1213.
   Android: Set the command line to --variations-pr=1213 in debug menu, restart the browser.
   iOS: Set Variations PR to 1213 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/1213@7475e4750dd1e1bfbca5e216549857553f9fa956
Uploaded	Fri, 20 Sep 2024 06:30:35 GMT
PR commit	ad3d0c2
Base commit	e7bfa3e
Merge commit	7475e47
Serial number	9eebfedca7052689e4c1d9ae3a022e6d

[kjozwiak]

Release verification(s)

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

Brave | 1.70.117 Chromium: 129.0.6668.59 (Official Build) (64-bit)
-- | --
Revision | 5d4da8332eb061f79bb47ec5b0bf595600fdcd55
OS | Windows 11 Version 23H2 (Build 22631.4169)

WorkaroundNewWindowFlash:Enabled Case - PASSED

• launched 1.70.117 Chromium: 129.0.6668.59 using brave.exe --enable-logging=stderr --variations-pr=1178
• relaunched once again using brave.exe --enable-logging=stderr --variations-pr=1178
• ensured that pull/1213@7475e4750dd1e1bfbca5e216549857553f9fa956 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

WorkaroundNewWindowFlash:Default Case - PASSED

• launched 1.70.117 Chromium: 129.0.6668.59 using brave.exe --enable-logging=stderr --variations-pr=1178
• relaunched once again using brave.exe --enable-logging=stderr --variations-pr=1178
• ensured that pull/1213@7475e4750dd1e1bfbca5e216549857553f9fa956 is appearing under brave://version
• ensured that WorkaroundNewWindowFlash:Disabled 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

[kjozwiak]

Uplift into main approved 👍 QA has verified the PR on Release as per #1213 (comment). The above is already fixed via client side for both Nightly & BETA.