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.cc

@@ -0,0 +1,54 @@
+// 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.
+
+#include "libcef/browser/global_preference_manager_impl.h"
+
+#include "libcef/browser/context.h"
+#include "libcef/browser/prefs/pref_helper.h"
+#include "libcef/browser/thread_util.h"
+
+#include "chrome/browser/browser_process.h"
+
+bool CefGlobalPreferenceManagerImpl::HasPreference(const CefString& name) {
+  CEF_REQUIRE_UIT_RETURN(false);
+  return pref_helper::HasPreference(g_browser_process->local_state(), name);
+}
+
+CefRefPtr<CefValue> CefGlobalPreferenceManagerImpl::GetPreference(
+    const CefString& name) {
+  CEF_REQUIRE_UIT_RETURN(nullptr);
+  return pref_helper::GetPreference(g_browser_process->local_state(), name);
+}
+
+CefRefPtr<CefDictionaryValue> CefGlobalPreferenceManagerImpl::GetAllPreferences(
+    bool include_defaults) {
+  CEF_REQUIRE_UIT_RETURN(nullptr);
+  return pref_helper::GetAllPreferences(g_browser_process->local_state(),
+                                        include_defaults);
+}
+
+bool CefGlobalPreferenceManagerImpl::CanSetPreference(const CefString& name) {
+  CEF_REQUIRE_UIT_RETURN(false);
+  return pref_helper::CanSetPreference(g_browser_process->local_state(), name);
+}
+
+bool CefGlobalPreferenceManagerImpl::SetPreference(const CefString& name,
+                                                   CefRefPtr<CefValue> value,
+                                                   CefString& error) {
+  CEF_REQUIRE_UIT_RETURN(false);
+  return pref_helper::SetPreference(g_browser_process->local_state(), name,
+                                    value, error);
+}
+
+// static
+CefRefPtr<CefPreferenceManager>
+CefPreferenceManager::GetGlobalPreferenceManager() {
+  // Verify that the context is in a valid state.
+  if (!CONTEXT_STATE_VALID()) {
+    NOTREACHED() << "context not valid";
+    return nullptr;
+  }
+
+  return new CefGlobalPreferenceManagerImpl();
+}