Support registration of custom preferences.

Custom global and request context preferences can now be registered via
CefBrowserProcessHandler::OnRegisterCustomPreferences. CefRequestContext
now extends CefPreferenceManager and global preferences can be accessed
via CefPreferenceManager::GetGlobalPreferenceManager.

libcef/browser/prefs/pref_helper.h

@@ -0,0 +1,33 @@
+// Copyright (c) 2022 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_PREFS_PREF_HELPER_H_
+#define CEF_LIBCEF_BROWSER_PREFS_PREF_HELPER_H_
+
+#include "include/cef_values.h"
+
+class PrefService;
+
+namespace pref_helper {
+
+// Function names and arguments match the CefPreferenceManager interface.
+
+bool HasPreference(PrefService* pref_service, const CefString& name);
+
+CefRefPtr<CefValue> GetPreference(PrefService* pref_service,
+                                  const CefString& name);
+
+CefRefPtr<CefDictionaryValue> GetAllPreferences(PrefService* pref_service,
+                                                bool include_defaults);
+
+bool CanSetPreference(PrefService* pref_service, const CefString& name);
+
+bool SetPreference(PrefService* pref_service,
+                   const CefString& name,
+                   CefRefPtr<CefValue> value,
+                   CefString& error);
+
+}  // namespace pref_helper
+
+#endif  // CEF_LIBCEF_BROWSER_PREFS_PREF_HELPER_H_