aura: Use PlatformWindow from WindowTreeHost.

ui/platform_window/win/win_window.h

Index: ui/platform_window/win/win_window.h
diff --git a/ui/platform_window/win/win_window.h b/ui/platform_window/win/win_window.h
index 644bd51dba3425d93a332a69a50b1f2e02c7cc59..0c0ac7baa9c3758a86ffd4210ef41fe3b486fd10 100644
--- a/ui/platform_window/win/win_window.h
+++ b/ui/platform_window/win/win_window.h
@@ -72,6 +72,15 @@ class WIN_WINDOW_EXPORT WinWindow : public NON_EXPORTED_BASE(PlatformWindow),
   DISALLOW_COPY_AND_ASSIGN(WinWindow);
 };
 
+namespace test {
+
+// Set true to let WindowTreeHostWin use a popup window
+// with no frame/title so that the window size and test's
+// expectations matches.
+WIN_WINDOW_EXPORT void SetUsePopupAsRootWindowForTest(bool use);
+
+}  // namespace test
+
 }  // namespace ui
 
 #endif  // UI_PLATFORM_WINDOW_WIN_WIN_WINDOW_H_