Add a shared memory variant of CefProcessMessage (fixes issue #3126)

CefSharedProcessMessageBuilder supports creation of a CefProcessMessage
backed by a CefSharedMemoryRegion.

Performance tests comparing the existing ArgumentList approach and the new
SharedMemoryRegion approach have been added to cefclient at
http://tests/ipc_performance.

CefMessageRouter has been updated to use SharedMemoryRegion as transport
for larger message payloads. The threshold is configurable via
|CefMessageRouterConfig.message_size_threshold|.

To test:
run `ceftests --gtest_filter=SendSharedProcessMessageTest.*:SharedProcessMessageTest.*:MessageRouterTest.Threshold*`

tests/cefclient/renderer/ipc_performance_test.h

@@ -0,0 +1,19 @@
+// 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_TESTS_CEFCLIENT_RENDERER_IPC_PERFORMANCE_TEST_H_
+#define CEF_TESTS_CEFCLIENT_RENDERER_IPC_PERFORMANCE_TEST_H_
+#pragma once
+
+#include "tests/shared/renderer/client_app_renderer.h"
+
+namespace client {
+namespace ipc_performance_test {
+
+void CreateDelegates(ClientAppRenderer::DelegateSet& delegates);
+
+}  // namespace ipc_performance_test
+}  // namespace client
+
+#endif  // CEF_TESTS_CEFCLIENT_RENDERER_IPC_PERFORMANCE_TEST_H_