mash: Migrate ScopedTargetRootWindow to //ash/common

ash/common/wm_shell.h

Index: ash/common/wm_shell.h
diff --git a/ash/common/wm_shell.h b/ash/common/wm_shell.h
index 79968ecf6f5c37f1942d14d18ca44d4879a8d057..ebaf5fadccfe247b7c2c982b9c67b6daab10df3c 100644
--- a/ash/common/wm_shell.h
+++ b/ash/common/wm_shell.h
@@ -111,6 +111,15 @@ class ASH_EXPORT WmShell {
     return new_window_delegate_.get();
   }
 
+  WmWindow* root_window_for_new_windows() {
+    return root_window_for_new_windows_;
+  }
+
+  // NOTE: Prefer ScopedRootWindowForNewWindows when setting temporarily.
+  void set_root_window_for_new_windows(WmWindow* root) {
+    root_window_for_new_windows_ = root;
+  }
+
   ShelfDelegate* shelf_delegate() { return shelf_delegate_.get(); }
 
   ShelfModel* shelf_model() { return shelf_model_.get(); }
@@ -145,11 +154,6 @@ class ASH_EXPORT WmShell {
   // Returns the root window for the specified display.
   virtual WmWindow* GetRootWindowForDisplayId(int64_t display_id) = 0;
 
-  // Returns the root window that newly created windows should be added to.
-  // NOTE: this returns the root, newly created window should be added to the
-  // appropriate container in the returned window.
-  virtual WmWindow* GetRootWindowForNewWindows() = 0;
-
   // Retuns the display info associated with |display_id|.
   // TODO(msw): Remove this when DisplayManager has been moved. crbug.com/622480
   virtual const DisplayInfo& GetDisplayInfo(int64_t display_id) const = 0;
@@ -349,6 +353,11 @@ class ASH_EXPORT WmShell {
 
   base::ObserverList<LockStateObserver> lock_state_observers_;
 
+  // The root window that newly created windows should be added to.
+  // NOTE: This is the root, so newly created windows should be added to the
+  // appropriate container inside this window.
+  WmWindow* root_window_for_new_windows_ = nullptr;
+
   bool simulate_modal_window_open_for_testing_ = false;
 
 #if defined(OS_CHROMEOS)