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/global_preference_manager_impl.h

@@ -0,0 +1,35 @@
+// 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_GLOBAL_PREFERENCE_MANAGER_IMPL_H_
+#define CEF_LIBCEF_BROWSER_GLOBAL_PREFERENCE_MANAGER_IMPL_H_
+#pragma once
+
+#include "include/cef_preference.h"
+
+// Implementation of the CefPreferenceManager interface for global preferences.
+class CefGlobalPreferenceManagerImpl : public CefPreferenceManager {
+ public:
+  CefGlobalPreferenceManagerImpl() = default;
+
+  CefGlobalPreferenceManagerImpl(const CefGlobalPreferenceManagerImpl&) =
+      delete;
+  CefGlobalPreferenceManagerImpl& operator=(
+      const CefGlobalPreferenceManagerImpl&) = delete;
+
+  // CefPreferenceManager methods.
+  bool HasPreference(const CefString& name) override;
+  CefRefPtr<CefValue> GetPreference(const CefString& name) override;
+  CefRefPtr<CefDictionaryValue> GetAllPreferences(
+      bool include_defaults) override;
+  bool CanSetPreference(const CefString& name) override;
+  bool SetPreference(const CefString& name,
+                     CefRefPtr<CefValue> value,
+                     CefString& error) override;
+
+ private:
+  IMPLEMENT_REFCOUNTING(CefGlobalPreferenceManagerImpl);
+};
+
+#endif  // CEF_LIBCEF_BROWSER_GLOBAL_PREFERENCE_MANAGER_IMPL_H_