[MD] Implement incognito colors as a NativeTheme (for Aura).

ui/views/views_delegate.h

Index: ui/views/views_delegate.h
diff --git a/ui/views/views_delegate.h b/ui/views/views_delegate.h
index 16ecca03ce7f543c5f76dc7e994642fdaf0f883e..4fc8a5c855d196548d5180ee35b6e03e99209d42 100644
--- a/ui/views/views_delegate.h
+++ b/ui/views/views_delegate.h
@@ -38,6 +38,7 @@ class Rect;
 
 namespace ui {
 class ContextFactory;
+class NativeTheme;
 }
 
 namespace views {
@@ -191,6 +192,10 @@ class VIEWS_EXPORT ViewsDelegate {
   // Returns a blocking pool task runner given a TaskRunnerType.
   virtual scoped_refptr<base::TaskRunner> GetBlockingPoolTaskRunner();
 
+  // Returns a value which, if non-null, will be used as the NativeTheme for
+  // |widget| in place of the platform default native theme.
+  virtual ui::NativeTheme* GetNativeThemeOverride(const Widget* widget);
+
  protected:
   ViewsDelegate();