Use WS_POPUP for ash_unittests

ui/aura/root_window_host_win.h

Index: ui/aura/root_window_host_win.h
diff --git a/ui/aura/root_window_host_win.h b/ui/aura/root_window_host_win.h
index f9ba6e02e6dae918b642240c3d4341d433df15e2..5b1a368c5323514d1027864a895bf980fa1c6133 100644
--- a/ui/aura/root_window_host_win.h
+++ b/ui/aura/root_window_host_win.h
@@ -7,6 +7,7 @@
 
 #include "base/compiler_specific.h"
 #include "ui/aura/root_window_host.h"
+#include "ui/base/ui_export.h"
 #include "ui/base/win/window_impl.h"
 
 namespace aura {
@@ -88,6 +89,15 @@ class RootWindowHostWin : public RootWindowHost, public ui::WindowImpl {
   DISALLOW_COPY_AND_ASSIGN(RootWindowHostWin);
 };
 
+namespace test {
+
+// Set true to let RootWindowHostWin use a popup window
+// with no frame/title so that the window size and test's
+// expectations matches.
+AURA_EXPORT void SetUsePopupAsRootWindowForTest(bool use);
+
+}  // namespace
+
 }  // namespace aura
 
 #endif  // UI_AURA_ROOT_WINDOW_HOST_WIN_H_