mash: Migrate ScopedTargetRootWindow to //ash/common

ash/aura/wm_shell_aura.cc

Index: ash/aura/wm_shell_aura.cc
diff --git a/ash/aura/wm_shell_aura.cc b/ash/aura/wm_shell_aura.cc
index 29af4b96cb5e2fec943568fa8f1b110694aab52f..b08ee84ded451c07c854fecab5c53f6f88e0d3e6 100644
--- a/ash/aura/wm_shell_aura.cc
+++ b/ash/aura/wm_shell_aura.cc
@@ -6,6 +6,7 @@
 
 #include <utility>
 
+#include "ash/aura/scoped_root_window_for_new_windows_aura.h"
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/shell_delegate.h"
@@ -91,6 +92,12 @@ WmWindow* WmShellAura::GetRootWindowForNewWindows() {
   return WmWindowAura::Get(Shell::GetTargetRootWindow());
 }
 
+std::unique_ptr<ScopedRootWindowForNewWindows>
+WmShellAura::CreateScopedRootWindowForNewWindows(WmWindow* root) {
+  return base::MakeUnique<ScopedRootWindowForNewWindowsAura>(
+      WmWindowAura::GetAuraWindow(root));
+}
+
 const DisplayInfo& WmShellAura::GetDisplayInfo(int64_t display_id) const {
   return Shell::GetInstance()->display_manager()->GetDisplayInfo(display_id);
 }