[Fallback Icons] Refactor FallbackIconService to be a BrowserContext-level singleton

chrome/browser/favicon/chrome_fallback_icon_client_factory.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_FAVICON_CHROME_FALLBACK_ICON_CLIENT_FACTORY_H_
+#define CHROME_BROWSER_FAVICON_CHROME_FALLBACK_ICON_CLIENT_FACTORY_H_
+
+#include "components/keyed_service/content/browser_context_keyed_service_factory.h"
+
+template <typename T> struct DefaultSingletonTraits;
+
+class FallbackIconClient;
+class Profile;
+
+// Singleton that owns all ChromeFallbackIconClients and associates them with
+// Profiles.
+class ChromeFallbackIconClientFactory
+    : public BrowserContextKeyedServiceFactory {
+ public:
+  // Returns the instance of FallbackIconClient associated with this profile
+  // (creating one if none exists).
+  static FallbackIconClient* GetForProfile(Profile* profile);
+
+  // Returns an instance of the factory singleton.
+  static ChromeFallbackIconClientFactory* GetInstance();
+
+ private:
+  friend struct DefaultSingletonTraits<ChromeFallbackIconClientFactory>;
+
+  ChromeFallbackIconClientFactory();
+  ~ChromeFallbackIconClientFactory() override;
+
+  // BrowserContextKeyedServiceFactory:
+  KeyedService* BuildServiceInstanceFor(
+      content::BrowserContext* profile) const override;
+  content::BrowserContext* GetBrowserContextToUse(
+      content::BrowserContext* context) const override;
+};
+
+#endif  // CHROME_BROWSER_FAVICON_CHROME_FALLBACK_ICON_CLIENT_FACTORY_H_