aura: Use PlatformWindow from WindowTreeHost.

ash/test/ash_test_base.cc

Index: ash/test/ash_test_base.cc
diff --git a/ash/test/ash_test_base.cc b/ash/test/ash_test_base.cc
index bd31bcc720ac4e6ba978ce4352d908aa0e5b37ea..3bf53dd1075c8de3693540e4273e0504ad48e8eb 100644
--- a/ash/test/ash_test_base.cc
+++ b/ash/test/ash_test_base.cc
@@ -44,6 +44,7 @@
 #include "base/win/windows_version.h"
 #include "ui/aura/remote_window_tree_host_win.h"
 #include "ui/aura/window_tree_host_win.h"
+#include "ui/platform_window/win/win_window.h"
 #include "win8/test/test_registrar_constants.h"
 #endif
 
@@ -123,7 +124,7 @@ void AshTestBase::SetUp() {
         switches::kAshHostWindowBounds, "1+1-800x600");
   }
 #if defined(OS_WIN)
-  aura::test::SetUsePopupAsRootWindowForTest(true);
+  ui::test::SetUsePopupAsRootWindowForTest(true);
 #endif
   ash_test_helper_->SetUp(start_session_);
 
@@ -173,7 +174,7 @@ void AshTestBase::TearDown() {
 
   ash_test_helper_->TearDown();
 #if defined(OS_WIN)
-  aura::test::SetUsePopupAsRootWindowForTest(false);
+  ui::test::SetUsePopupAsRootWindowForTest(false);
   // Kill the viewer process if we spun one up.
   metro_viewer_host_.reset();