Moves maintaining ShellObservers back to Shell

ash/common/system/overview/overview_button_tray.cc

Index: ash/common/system/overview/overview_button_tray.cc
diff --git a/ash/common/system/overview/overview_button_tray.cc b/ash/common/system/overview/overview_button_tray.cc
index c28d814dd6ffe63a28b7d2901866606108e14d4a..920948bb984b6740a3bfbb24361acc2883a1f525 100644
--- a/ash/common/system/overview/overview_button_tray.cc
+++ b/ash/common/system/overview/overview_button_tray.cc
@@ -12,6 +12,7 @@
 #include "ash/common/wm/overview/window_selector_controller.h"
 #include "ash/common/wm_shell.h"
 #include "ash/resources/vector_icons/vector_icons.h"
+#include "ash/shell.h"
 #include "ash/strings/grit/ash_strings.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/gfx/paint_vector_icon.h"
@@ -33,12 +34,12 @@ OverviewButtonTray::OverviewButtonTray(WmShelf* wm_shelf)
   // horizontal shelf, no separator is required.
   set_separator_visibility(false);
 
-  WmShell::Get()->AddShellObserver(this);
+  Shell::GetInstance()->AddShellObserver(this);
   WmShell::Get()->GetSessionStateDelegate()->AddSessionStateObserver(this);
 }
 
 OverviewButtonTray::~OverviewButtonTray() {
-  WmShell::Get()->RemoveShellObserver(this);
+  Shell::GetInstance()->RemoveShellObserver(this);
   WmShell::Get()->GetSessionStateDelegate()->RemoveSessionStateObserver(this);
 }