Move yet more from WmShell to Shell

ash/common/session/session_state_observer.cc

Index: ash/common/session/session_state_observer.cc
diff --git a/ash/common/session/session_state_observer.cc b/ash/common/session/session_state_observer.cc
index 709a92da27cb27153e1db0717bb1e0e7395d54b7..303a2c0ebf37bcacc9e7e59824e87b9511b3aaa4 100644
--- a/ash/common/session/session_state_observer.cc
+++ b/ash/common/session/session_state_observer.cc
@@ -5,20 +5,19 @@
 #include "ash/common/session/session_state_observer.h"
 
 #include "ash/common/session/session_controller.h"
-#include "ash/common/wm_shell.h"
+#include "ash/shell.h"
 
 namespace ash {
 
 ScopedSessionStateObserver::ScopedSessionStateObserver(
     SessionStateObserver* observer)
     : observer_(observer) {
-  WmShell::Get()->session_controller()->AddSessionStateObserver(observer_);
+  Shell::Get()->session_controller()->AddSessionStateObserver(observer_);
 }
 
 ScopedSessionStateObserver::~ScopedSessionStateObserver() {
-  if (WmShell::Get()) {
-    WmShell::Get()->session_controller()->RemoveSessionStateObserver(observer_);
-  }
+  if (Shell::HasInstance())
+    Shell::Get()->session_controller()->RemoveSessionStateObserver(observer_);
 }
 
 }  // namespace ash