Refactor extension app icon.

chrome/browser/extensions/extension_app_icon_delegate.h

+// Copyright 2017 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 CHROME_BROWSER_EXTENSIONS_EXTENSION_APP_ICON_DELEGATE_H_
+#define CHROME_BROWSER_EXTENSIONS_EXTENSION_APP_ICON_DELEGATE_H_
+
+namespace extensions {
+
+class ExtensionAppIcon;
+
+class ExtensionAppIconDelegate {
+ public:
+  // Invoked when ExtensionAppIcon is updated. "icon->image_skia()" contains
+  // the current icon image.
+  virtual void OnIconUpdated(ExtensionAppIcon* icon) = 0;
+
+ protected:
+  virtual ~ExtensionAppIconDelegate() {}
+};
+
+}  // namespace extensions
+
+#endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_APP_ICON_DELEGATE_H_