The ShellDesktopControllerAuraTest test (InputEvents, in particular), create an aura::WindowTreeHost that out-lives the display::Screen.

In particular, the display::Screen is reset with the stack
  display::Screen::SetScreenInstance()
  extensions::ShellDesktopControllerAura::DestroyRootWindow()
  extensions::ShellDesktopControllerAura::~ShellDesktopControllerAura()
  extensions::ShellDesktopControllerAura::~ShellDesktopControllerAura()
  extensions::ShellDesktopControllerAuraTest::TearDown()

Shortly thereafter, we try to destroy an aura::WindowTreeHost at
  aura::WindowTreeHost::~WindowTreeHost()
  aura::WindowTreeHostX11::~WindowTreeHostX11()
  aura::WindowTreeHostX11::~WindowTreeHostX11()
  aura::test::AuraTestHelper::TearDown()
  aura::test::AuraTestBase::TearDown()
  extensions::ShellDesktopControllerAuraTest::TearDown()

The aura::WindowTreeHost regularly reaches into display::Screen, and is also now an observer to display changes.

This puts us in the messy situation of trying to un-become a DisplayObserver after display::Screen has been destroyed.