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

ui/views/views_delegate.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_VIEWS_VIEWS_DELEGATE_H_
 #define UI_VIEWS_VIEWS_DELEGATE_H_
 
 #include <string>
 
 #if defined(OS_WIN)
@@ skipping 20 matching lines @@
 class SiteInstance;
 }
 
 namespace gfx {
 class ImageSkia;
 class Rect;
 }
 
 namespace ui {
 class ContextFactory;
+class NativeTheme;
 }
 
 namespace views {
 
 class NativeWidget;
 class NonClientFrameView;
 class ViewsTouchEditingControllerFactory;
 class View;
 class Widget;
 
@@ skipping 133 matching lines @@
   // not changed, |callback| is never run.
   //
   // The return value is a bitmask of AppbarAutohideEdge.
   virtual int GetAppbarAutohideEdges(HMONITOR monitor,
                                      const base::Closure& callback);
 #endif
 
   // 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* GetNonDefaultNativeTheme(const Widget* widget);

[Peter Kasting, 2015/11/11 00:34:46]

Can this return the platform default native theme in cases where there's no
override?  If so, we can just name this GetNativeTheme() and maybe always use it
from Widget?

If not, maybe GetNativeThemeOverride() would be slightly better, since
"non-default" implies it's returning some sort of secondary theme so that
callers can use both, or something strange.

[Evan Stade, 2015/11/11 01:05:32]

I don't know how we'd do this because ViewsDelegate::GetInstance() can
apparently be null (also, adding implementation to the base ViewsDelegate seems
bad, and adding it to ChromeViewsDelegate is impossible because it needs access
to internal apis).
I'm fine with the Override name.

[Peter Kasting, 2015/11/11 01:25:47]

I meant for Widget to so something like:

  return views_delegate ? views_delegate->GetNativeTheme() : GetNativeTheme();

So we'd still need to null-check, it's just that we wouldn't need to also check
the return value from this.
If it's technically infeasible, OK.  I wasn't sure.

+
  protected:
   ViewsDelegate();
 
  private:
   scoped_ptr<ViewsTouchEditingControllerFactory> views_tsc_factory_;
 
 #if defined(USE_AURA)
   scoped_ptr<TouchSelectionMenuRunnerViews> touch_selection_menu_runner_;
 #endif
 
   NativeWidgetFactory native_widget_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ViewsDelegate);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_VIEWS_DELEGATE_H_