Change name of NativeTheme::instance() to make it clear it's web only.

ui/views/view.h

Index: ui/views/view.h
diff --git a/ui/views/view.h b/ui/views/view.h
index 9d43e2cea038fb87a009c9a7a41423c2650bd91f..8de66635891eb0dd8a999bdf70e67704d18a2541 100644
--- a/ui/views/view.h
+++ b/ui/views/view.h
@@ -513,8 +513,9 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   ui::ThemeProvider* GetThemeProvider() const;
 
   // Returns the NativeTheme to use for this View. This calls through to
-  // GetNativeTheme() on the Widget this View is in. If this View is not in a
-  // Widget this returns ui::NativeTheme::instance().
+  // GetNativeTheme() on the Widget this View is in, or provides a default
+  // theme if there's no widget. Warning: the default theme might not be
+  // correct; you should probably override OnNativeThemeChanged().
   ui::NativeTheme* GetNativeTheme() {
     return const_cast<ui::NativeTheme*>(
         const_cast<const View*>(this)->GetNativeTheme());