[Rounded Corners]: In rounded corners mode, there's no separation between the web_contents and the browser chrome

[fallaciousreasoning]

1. In light mode, with brave://flags/#brave-web-view-rounded-corners enabled
2. Navigate to https://google.com

Expected:
There is some kind of separation between the webcontents and the browser chrome

Actual:
There is not:

[simonhong]

I can see space between web contents and sidebar in any theme.

When vertical tab is enabled, there is no left-side space of web contents. I think this is the issue.
Same issue when vertical tab is on right side.

[fallaciousreasoning]

@simonhong yeah you're right - the issue was there were no margins around the webcontents - I've added them in brave/brave-core#25738. Mind taking a look and seeing what you think?