Handle app custom icon via aura::Window property.

extensions/browser/app_window/app_delegate.h

 // Copyright 2014 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 EXTENSIONS_BROWSER_APP_WINDOW_APP_DELEGATE_H_
 #define EXTENSIONS_BROWSER_APP_WINDOW_APP_DELEGATE_H_
 
 #include "base/callback_forward.h"
 #include "content/public/common/media_stream_request.h"
 #include "third_party/skia/include/core/SkColor.h"
@@ skipping 52 matching lines @@
                               const content::FileChooserParams& params) = 0;
   virtual void RequestMediaAccessPermission(
       content::WebContents* web_contents,
       const content::MediaStreamRequest& request,
       const content::MediaResponseCallback& callback,
       const Extension* extension) = 0;
   virtual bool CheckMediaAccessPermission(content::WebContents* web_contents,
                                           const GURL& security_origin,
                                           content::MediaStreamType type,
                                           const Extension* extension) = 0;
-  virtual int PreferredIconSize() = 0;
+  virtual int PreferredIconSize() const = 0;
 
   // Web contents modal dialog support.
   virtual void SetWebContentsBlocked(content::WebContents* web_contents,
                                      bool blocked) = 0;
   virtual bool IsWebContentsVisible(content::WebContents* web_contents) = 0;
 
   // |callback| will be called when the process is about to terminate.
   virtual void SetTerminatingCallback(const base::Closure& callback) = 0;
 
   // Called when the app is hidden or shown.
   virtual void OnHide() = 0;
   virtual void OnShow() = 0;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_APP_WINDOW_APP_DELEGATE_H_