[MD settings] advanced font settings extension link

chrome/browser/ui/webui/settings/font_handler.h

 // Copyright 2015 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_UI_WEBUI_SETTINGS_FONT_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_SETTINGS_FONT_HANDLER_H_
 
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
+#include "base/scoped_observer.h"
 #include "chrome/browser/ui/webui/settings/settings_page_ui_handler.h"
+#include "extensions/browser/extension_registry_observer.h"
 
 namespace base {
 class ListValue;
 }
 
 namespace content {
 class WebUI;
 }
 
+namespace extensions {
+class Extension;
+class ExtensionRegistry;
+}
+
 class Profile;
 
 namespace settings {
 
 // Handle OS font list, font preference settings and character encoding.
-class FontHandler : public SettingsPageUIHandler {
+class FontHandler : public SettingsPageUIHandler,
+                    public extensions::ExtensionRegistryObserver {
  public:
   explicit FontHandler(content::WebUI* webui);
   ~FontHandler() override;
 
   // SettingsPageUIHandler implementation.
   void RegisterMessages() override;
+  void RenderViewReused() override;
+
+  // ExtensionRegistryObserver implementation.
+  void OnExtensionLoaded(content::BrowserContext* browser_context,
+                         const extensions::Extension* extension) override;
+  void OnExtensionUnloaded(
+      content::BrowserContext* browser_context,
+      const extensions::Extension* extension,
+      extensions::UnloadedExtensionInfo::Reason reason) override;
 
  private:
   // Handler for script asking for font information.
   void HandleFetchFontsData(const base::ListValue* args);
 
+  // Listen for changes to whether the advanced font extension is available.
+  // An initial update will be sent when observation begins.
+  void HandleObserveAdvancedFontExtensionAvailable(const base::ListValue* args);
+
+  // Open the advanced font settings page.
+  void HandleOpenAdvancedFontSettings(const base::ListValue* args);
+
   // Callback to handle fonts loading.
   void FontListHasLoaded(std::string callback_id,
                          std::unique_ptr<base::ListValue> list);
 
+  const extensions::Extension* GetAdvancedFontSettingsExtension();
+
+  void NotifyAdvancedFontSettingsAvailability();
+
+  ScopedObserver<extensions::ExtensionRegistry,
+                 extensions::ExtensionRegistryObserver>
+      extension_registry_observer_;
+
   Profile* profile_;  // Weak pointer.
 
   base::WeakPtrFactory<FontHandler> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(FontHandler);
 };
 
 }  // namespace settings
 
 #endif  // CHROME_BROWSER_UI_WEBUI_SETTINGS_FONT_HANDLER_H_