chromeos: Fix shelf appearing at login screen under mash

ash/common/wm_shell.cc

Index: ash/common/wm_shell.cc
diff --git a/ash/common/wm_shell.cc b/ash/common/wm_shell.cc
index 7f4f6838c81b86ac8ba0a2454a7f6970164a5456..0bd31c79726c93aee3144a0d5907229ba092c74d 100644
--- a/ash/common/wm_shell.cc
+++ b/ash/common/wm_shell.cc
@@ -151,11 +151,6 @@ void WmShell::CreateShelf() {
     root_window->GetRootWindowController()->CreateShelf();
 }
 
-void WmShell::ShowShelf() {
-  for (WmWindow* root_window : GetAllRootWindows())
-    root_window->GetRootWindowController()->ShowShelf();
-}
-
 void WmShell::CreateShelfDelegate() {
   // May be called multiple times as shelves are created and destroyed.
   if (shelf_delegate_)
@@ -272,9 +267,12 @@ WmShell::WmShell(std::unique_ptr<ShellDelegate> shell_delegate)
   keyboard_brightness_control_delegate_.reset(new KeyboardBrightnessController);
   vpn_list_ = base::MakeUnique<VpnList>();
 #endif
+  session_controller_->AddSessionStateObserver(this);
 }
 
-WmShell::~WmShell() {}
+WmShell::~WmShell() {
+  session_controller_->RemoveSessionStateObserver(this);
+}
 
 WmRootWindowController* WmShell::GetPrimaryRootWindowController() {
   return GetPrimaryRootWindow()->GetRootWindowController();
@@ -415,4 +413,11 @@ void WmShell::SetAcceleratorController(
   accelerator_controller_ = std::move(accelerator_controller);
 }
 
+void WmShell::SessionStateChanged(session_manager::SessionState state) {
+  // Create the shelf when a session becomes active. It's safe to do this
+  // multiple times (e.g. initial login vs. multiprofile add session).
+  if (state == session_manager::SessionState::ACTIVE)
+    CreateShelf();
+}
+
 }  // namespace ash