F/U PR for #35616
Previously, Brave suppressed the upstream SidePanelHeader by iterating
all registry entries and calling set_should_show_header(false) at the
point when the global registry (SidePanelUtil::PopulateGlobalEntries)
and contextual registry (brave_side_panel_utils::RegisterContextualSidePanel)
were first populated. This missed entries registered later, such as
kCustomizeChrome, which registers its SidePanelEntry in DidFinishNavigation
after the initial registry setup loop had already run.
Move the call to BraveSidePanelCoordinator::PopulateSidePanel instead,
which is invoked unconditionally for every entry at the moment it is
displayed. This guarantees the upstream header is suppressed regardless
of when or how the entry was registered.
Also extend SidebarV2NoUpstreamHeaderTest to verify that the
CustomizeChrome panel does not receive an upstream header.
Simon Hong
1dc0672cff