Resolves brave/brave-browser#53555 - GetTopSeparatorType(): return kNone when toolbar and bookmark bar are not visible so no separator is drawn at the top of the contents view in fullscreen. - chromium_src: define SetDelegateForTesting (declared but not defined upstream) so tests can inject a layout delegate. TEST=BraveBrowserViewTabbedLayoutImplTest.GetTopSeparatorTypeNoneWhenNoVisibleTopUI