mash: Migrate wallpaper controllers to ash/common.

ash/common/wm_shell.cc

Index: ash/common/wm_shell.cc
diff --git a/ash/common/wm_shell.cc b/ash/common/wm_shell.cc
index c98fa1dbc15e3e32cd1d67fec634d25dd54f3742..3e73becbaab11fd408c019b61c0089c269afc8c7 100644
--- a/ash/common/wm_shell.cc
+++ b/ash/common/wm_shell.cc
@@ -25,6 +25,7 @@
 #include "ash/common/system/toast/toast_manager.h"
 #include "ash/common/system/tray/system_tray_delegate.h"
 #include "ash/common/system/tray/system_tray_notifier.h"
+#include "ash/common/wallpaper/wallpaper_controller.h"
 #include "ash/common/wallpaper/wallpaper_delegate.h"
 #include "ash/common/wm/immersive_context_ash.h"
 #include "ash/common/wm/maximize_mode/maximize_mode_controller.h"
@@ -76,11 +77,15 @@ void WmShell::Initialize() {
   views::FocusManagerFactory::Install(new AshFocusManagerFactory);
 
   new_window_delegate_.reset(delegate_->CreateNewWindowDelegate());
+
+  wallpaper_controller_.reset(new WallpaperController(GetBlockingPool()));
 }
 
 void WmShell::Shutdown() {
-  // Accesses WmShell in its destructor.
+  // These members access WmShell in their destructors.
+  wallpaper_controller_.reset();
   accessibility_delegate_.reset();
+
   // ShelfWindowWatcher has window observers and a pointer to the shelf model.
   shelf_window_watcher_.reset();
   // ShelfItemDelegate subclasses it owns have complex cleanup to run (e.g. ARC