improve animation/rendering when resizing brave://welcome

[kjozwiak]

Description

Similar to #27061, when resizing brave://welcome, you'll notice that the extra space that was created is slow to render. You'll notice dark/black area's for a few ms before the screen renders correctly.

Steps to Reproduce

1. install & launch 1.48.57 Chromium: 108.0.5359.128
2. notice how smoothly brave://welcome loads due to Optimized the new Welcome UI and style fixes brave-core#16164
3. start resizing the screen and you'll run to similar issues mentioned via improve the animation/rendering of onboarding v2 #27061

Actual result:

Expected result:

Resizing brave://welcome should be a smoother experience. We shouldn't be showing parts of the screen as blank/dark because the rendering hasn't completed.

Reproduces how often:

100% reproducible when using the STR/Cases outlined above.

Brave version (brave://version info)

Brave | 1.48.57 Chromium: 108.0.5359.128 (Official Build) nightly (64-bit)
-- | --
Revision | 1cd27afdb8e5d057070c0961e04c490d2aca1aa0-refs/branch-heads/5359@{#1185}
OS | Windows 11 Version 22H2 (Build 22621.963)

Version/Channel Information:

• Can you reproduce this issue with the current release? No
• 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? N/A
• Does the issue resolve itself when disabling Brave Rewards? N/A
• Is the issue reproducible on the latest version of Chrome? N/A

Miscellaneous Information:

CCing @rebron @nullhook