Resolves https://github.com/brave/brave-browser/issues/49794
Security review: https://github.com/brave/reviews/issues/2113
We ues tab contents for web panel to make web panel contents have same security level with tab contents.
And `BraveMultiContentsView` provides another space for showing panel contents.
When web panel item is clicked, `SidebarWebPanelController::OpenWebPanel()`
creates pinned tab and set that tab's contents as web panel's contents.
`BraveMultiContentsView::contents_container_view_for_web_panel_` works as its holder.
Whenever its active stage changes, its focus border is also updated.
For now, same focus border is used with split view.
As we can have split view and web panel both at once, this could conflict with upstream's code.
Upstream assumes that current active tab is split tab when split view is opened.
But it couldn't with web panel.
To avoid that conflict, `BraveMultiContentsViewDelegateImpl` is introduced and
`BraveBrowserView::IsWebPanelContents()` is used to skip that conflict.
TEST=SidebarBrowserWithWebPanelTest.WebPanelTest
Simon Hong
e4657b8f98