Moves maintaining ShellObservers back to Shell

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index e6d4b4ec7e220d794fdfb4f2aecdd479c151c743..6e233915143310b9a1d5d3eddd35dcbac8b3710e 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -358,16 +358,6 @@ std::unique_ptr<KeyEventWatcher> WmShellMus::CreateKeyEventWatcher() {
   return std::unique_ptr<KeyEventWatcher>();
 }
 
-void WmShellMus::OnOverviewModeStarting() {
-  for (auto& observer : *shell_observers())
-    observer.OnOverviewModeStarting();
-}
-
-void WmShellMus::OnOverviewModeEnded() {
-  for (auto& observer : *shell_observers())
-    observer.OnOverviewModeEnded();
-}
-
 SessionStateDelegate* WmShellMus::GetSessionStateDelegate() {
   return session_state_delegate_
              ? session_state_delegate_.get()