Moves maintaining ShellObservers back to Shell

ash/aura/wm_shell_aura.cc

Index: ash/aura/wm_shell_aura.cc
diff --git a/ash/aura/wm_shell_aura.cc b/ash/aura/wm_shell_aura.cc
index b58a62841f182787275b7dbe8bcb8ba1b0a52a63..7400f34f88a86b812798f5fb84ee58f6b3ec110d 100644
--- a/ash/aura/wm_shell_aura.cc
+++ b/ash/aura/wm_shell_aura.cc
@@ -224,16 +224,6 @@ std::unique_ptr<KeyEventWatcher> WmShellAura::CreateKeyEventWatcher() {
   return base::MakeUnique<KeyEventWatcherAura>();
 }
 
-void WmShellAura::OnOverviewModeStarting() {
-  for (auto& observer : *shell_observers())
-    observer.OnOverviewModeStarting();
-}
-
-void WmShellAura::OnOverviewModeEnded() {
-  for (auto& observer : *shell_observers())
-    observer.OnOverviewModeEnded();
-}
-
 SessionStateDelegate* WmShellAura::GetSessionStateDelegate() {
   return Shell::GetInstance()->session_state_delegate();
 }