mash: Migrate wallpaper controllers to ash/common.

ash/mus/bridge/wm_shell_mus.h

Index: ash/mus/bridge/wm_shell_mus.h
diff --git a/ash/mus/bridge/wm_shell_mus.h b/ash/mus/bridge/wm_shell_mus.h
index de6ef61cd636fd0809b6b350e82b074e08e38822..ae9935ffcb88dc8c1945bf5c111697fa899eecfa 100644
--- a/ash/mus/bridge/wm_shell_mus.h
+++ b/ash/mus/bridge/wm_shell_mus.h
@@ -13,6 +13,7 @@
 #include "ash/common/wm_shell.h"
 #include "base/macros.h"
 #include "base/observer_list.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "services/ui/public/cpp/window_tree_client_observer.h"
 
 namespace shell {
@@ -112,6 +113,7 @@ class WmShellMus : public WmShell, public ui::WindowTreeClientObserver {
                          views::PointerWatcherEventTypes events) override;
   void RemovePointerWatcher(views::PointerWatcher* watcher) override;
   bool IsTouchDown() override;
+  base::SequencedWorkerPool* GetBlockingPool() override;
 #if defined(OS_CHROMEOS)
   void ToggleIgnoreExternalKeyboard() override;
 #endif
@@ -144,6 +146,8 @@ class WmShellMus : public WmShell, public ui::WindowTreeClientObserver {
 
   base::ObserverList<WmActivationObserver> activation_observers_;
 
+  scoped_refptr<base::SequencedWorkerPool> blocking_pool_;
+
   DISALLOW_COPY_AND_ASSIGN(WmShellMus);
 };