Moves maintaining ShellObservers back to Shell

ash/common/wm/overview/window_selector_controller.cc

Index: ash/common/wm/overview/window_selector_controller.cc
diff --git a/ash/common/wm/overview/window_selector_controller.cc b/ash/common/wm/overview/window_selector_controller.cc
index 2c8b4efa2cf05370fa66a9ea48442e2d6f419fd5..5f3152c351291bd155c667cdd1108e4b3e18ae31 100644
--- a/ash/common/wm/overview/window_selector_controller.cc
+++ b/ash/common/wm/overview/window_selector_controller.cc
@@ -13,6 +13,7 @@
 #include "ash/common/wm/window_state.h"
 #include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
+#include "ash/shell.h"
 #include "base/metrics/histogram_macros.h"
 
 namespace ash {
@@ -63,7 +64,7 @@ bool WindowSelectorController::ToggleOverview() {
     if (windows.empty())
       return false;
 
-    WmShell::Get()->OnOverviewModeStarting();
+    Shell::GetInstance()->OnOverviewModeStarting();
     window_selector_.reset(new WindowSelector(this));
     window_selector_->Init(windows);
     OnSelectionStarted();
@@ -96,7 +97,7 @@ void WindowSelectorController::OnSelectionEnded() {
   window_selector_->Shutdown();
   window_selector_.reset();
   last_selection_time_ = base::Time::Now();
-  WmShell::Get()->OnOverviewModeEnded();
+  Shell::GetInstance()->OnOverviewModeEnded();
 }
 
 void WindowSelectorController::AddDelayedAnimationObserver(