Makes a couple of tests reset state appropriately

ui/gfx/screen.h

Index: ui/gfx/screen.h
diff --git a/ui/gfx/screen.h b/ui/gfx/screen.h
index 44b58c3b9935050b314ad63c070c3fbe6b89ff13..65d78fe4a4f4f9b5683ec27bfab26dfa6d9ae897 100644
--- a/ui/gfx/screen.h
+++ b/ui/gfx/screen.h
@@ -37,6 +37,8 @@ class GFX_EXPORT Screen {
 
   // 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
@@ -45,6 +47,8 @@ class GFX_EXPORT Screen {
 
   // 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();