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

components/favicon/core/browser/fallback_icon_client.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 COMPONENTS_FAVICON_CORE_BROWSER_FALLBACK_ICON_CLIENT_H_
+#define COMPONENTS_FAVICON_CORE_BROWSER_FALLBACK_ICON_CLIENT_H_
+
+#include <string>
+#include <vector>
+
+#include "components/keyed_service/core/keyed_service.h"
+
+namespace favicon_base {
+class FallbackIconService;
+}  // namespace favicon_base
+
+// This class abstracts operations that depend on the embedder's environment,
+// e.g. Chrome.
+class FallbackIconClient : public KeyedService {
+ public:
+  ~FallbackIconClient() override {}
+
+  // Returns a list of font names for fallback icon rendering.
+  virtual const std::vector<std::string>& GetFontNameList() = 0;
+};
+
+#endif  // COMPONENTS_FAVICON_CORE_BROWSER_FALLBACK_ICON_CLIENT_H_