Add visualization for Chrome configuration changes (fixes #3892)

- Add new API to retrieve/observe configuration values.
- cefclient: Add https://tests/config to inspect configuration
  values in real time.

libcef/browser/setting_helper.h

@@ -0,0 +1,81 @@
+// Copyright (c) 2025 The Chromium Embedded Framework 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 CEF_LIBCEF_BROWSER_SETTING_HELPER_H_
+#define CEF_LIBCEF_BROWSER_SETTING_HELPER_H_
+
+#include "base/memory/raw_ptr.h"
+#include "base/observer_list.h"
+#include "cef/include/cef_registration.h"
+#include "components/content_settings/core/browser/content_settings_observer.h"
+
+class CefSettingObserver;
+class CefRegistration;
+class HostContentSettingsMap;
+
+namespace setting_helper {
+
+class Registration : public base::CheckedObserver {
+ public:
+  virtual void Detach() = 0;
+  virtual void RunCallback(const CefString& requesting_url,
+                           const CefString& top_level_url,
+                           cef_content_setting_types_t content_type) const = 0;
+};
+
+class RegistrationImpl;
+
+// Automatically manages the registration of one or more CefSettingObserver
+// objects with a HostContentSettingsMap. When the Registrar is destroyed, all
+// registered observers are automatically unregistered with the
+// HostContentSettingsMap. Loosely based on PrefChangeRegistrar.
+class Registrar final : public content_settings::Observer {
+ public:
+  Registrar() = default;
+
+  Registrar(const Registrar&) = delete;
+  Registrar& operator=(const Registrar&) = delete;
+
+  ~Registrar();
+
+  // Must be called before adding or removing observers. Can be called more
+  // than once as long as the value of |settings| doesn't change.
+  void Init(HostContentSettingsMap* settings);
+
+  // Removes all observers and clears the reference to the
+  // HostContentSettingsMap. `Init` must be called before adding or removing any
+  // observers.
+  void Reset();
+
+  // Removes all observers that have been previously added with a call to Add.
+  void RemoveAll();
+
+  // Returns true if no observers are registered.
+  bool IsEmpty() const;
+
+  // Adds a setting observer. All registered observers will be automatically
+  // unregistered and detached when the Registrar's destructor is called.
+  CefRefPtr<CefRegistration> AddObserver(
+      CefRefPtr<CefSettingObserver> observer);
+
+ private:
+  friend class RegistrationImpl;
+
+  void RemoveObserver(Registration* registration);
+
+  // content_settings::Observer:
+  void OnContentSettingChanged(
+      const ContentSettingsPattern& primary_pattern,
+      const ContentSettingsPattern& secondary_pattern,
+      ContentSettingsTypeSet content_type_set) override;
+
+  raw_ptr<HostContentSettingsMap, AcrossTasksDanglingUntriaged> settings_ =
+      nullptr;
+
+  base::ObserverList<Registration> observers_;
+};
+
+}  // namespace setting_helper
+
+#endif  // CEF_LIBCEF_BROWSER_SETTING_HELPER_H_