Fix Overlap of Scrollbar on "Developer mode" toggle in brave://extensions/

[Cleveho]

Steps to Reproduce

1. Open Brave
2. Visit brave://extensions/

Actual result:

Overlapping of Scrollbar on "Developer mode" toggle in brave://extensions/

Expected result:

Developer mode toggle not overlapped with Scrollbar

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave 1.36.109 Chromium: 99.0.4844.51 (Official Build) (64-bit)
Revision d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS Windows 11 Version 21H2 (Build 22000.493)

Version/Channel Information:

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

Miscellaneous Information:

Fix overlapping of scrollbar & make corners of Developer mode toggle circular

[GeetaSarvadnya]

@Cleveho Thanks for logging the issue. The issue is reproducible in windows 10 x64.

cc: @rebron