mash: Avoid shelf crashes on display removal.

ash/mus/root_window_controller.cc

Index: ash/mus/root_window_controller.cc
diff --git a/ash/mus/root_window_controller.cc b/ash/mus/root_window_controller.cc
index f9584af0e17b02428c77c52f56eab04970270d7f..4768f3f6ebdb689f1f389b73964464d84d111ca3 100644
--- a/ash/mus/root_window_controller.cc
+++ b/ash/mus/root_window_controller.cc
@@ -135,6 +135,10 @@ WmWindowMus* RootWindowController::GetWindowByShellWindowId(int id) {
       WmWindowMus::Get(root_)->GetChildByShellWindowId(id));
 }
 
+void RootWindowController::Shutdown() {
+  wm_shelf_->ShutdownShelfWidget();

[sky, 2016/09/01 23:51:17]

Can this be moved to WmRootWindowController? I think there is a bunch of stuff
in RootWindowController::CloseChildWindows that should likely move to
WmRootWindowController.

+}
+
 gfx::Rect RootWindowController::CalculateDefaultBounds(
     ui::Window* window) const {
   if (window->HasSharedProperty(