Makes a couple of tests reset state appropriately

ui/gfx/screen.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_SCREEN_H_
 #define UI_GFX_SCREEN_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 19 matching lines @@
   // Retrieves the Screen that the specified NativeView belongs to. A value of
   // NULL is treated as |SCREEN_TYPE_NATIVE|.
   static Screen* GetScreenFor(NativeView view);
 
   // Returns the SCREEN_TYPE_NATIVE Screen. This should be used with caution,
   // as it is likely to be incorrect for code that runs on Windows.
   static Screen* GetNativeScreen();
 
   // Sets the global screen for a particular screen type. Only the _NATIVE
   // ScreenType must be provided.
+  // NOTE: this does not take ownership of |screen|. Tests must be sure to
+  // reset any state they install.
   static void SetScreenInstance(ScreenType type, Screen* instance);
 
   // Returns the global screen for a particular type. Types other than _NATIVE
   // may be NULL.
   static Screen* GetScreenByType(ScreenType type);
 
   // Sets the global ScreenTypeDelegate. May be left unset if the platform
   // uses only the _NATIVE ScreenType.
+  // NOTE: this does not take ownership of |delegate|. Tests must be sure to
+  // reset any state they install.
   static void SetScreenTypeDelegate(ScreenTypeDelegate* delegate);
 
   Screen();
   virtual ~Screen();
 
   // Returns the current absolute position of the mouse pointer.
   virtual gfx::Point GetCursorScreenPoint() = 0;
 
   // Returns the window under the cursor.
   virtual gfx::NativeWindow GetWindowUnderCursor() = 0;
@@ skipping 31 matching lines @@
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Screen);
 };
 
 Screen* CreateNativeScreen();
 
 }  // namespace gfx
 
 #endif  // UI_GFX_SCREEN_H_