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*`

libcef/common/process_message_smr_impl.h

@@ -0,0 +1,58 @@
+// 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_COMMON_PROCESS_MESSAGE_SMR_IMPL_H_
+#define CEF_LIBCEF_COMMON_PROCESS_MESSAGE_SMR_IMPL_H_
+#pragma once
+
+#include "include/cef_process_message.h"
+#include "include/cef_shared_process_message_builder.h"
+
+#include "base/memory/read_only_shared_memory_region.h"
+
+class CefProcessMessageSMRImpl final : public CefProcessMessage {
+ public:
+  CefProcessMessageSMRImpl(const CefString& name,
+                           base::ReadOnlySharedMemoryRegion&& region);
+  CefProcessMessageSMRImpl(const CefProcessMessageSMRImpl&) = delete;
+  CefProcessMessageSMRImpl& operator=(const CefProcessMessageSMRImpl&) = delete;
+  ~CefProcessMessageSMRImpl() override;
+
+  // CefProcessMessage methods.
+  bool IsValid() override;
+  bool IsReadOnly() override { return true; }
+  CefRefPtr<CefProcessMessage> Copy() override { return nullptr; }
+  CefString GetName() override;
+  CefRefPtr<CefListValue> GetArgumentList() override { return nullptr; };
+  CefRefPtr<CefSharedMemoryRegion> GetSharedMemoryRegion() override;
+  [[nodiscard]] base::ReadOnlySharedMemoryRegion TakeRegion();
+
+ private:
+  const CefString name_;
+  base::ReadOnlySharedMemoryRegion region_;
+
+  IMPLEMENT_REFCOUNTING(CefProcessMessageSMRImpl);
+};
+
+class CefSharedProcessMessageBuilderImpl final
+    : public CefSharedProcessMessageBuilder {
+ public:
+  CefSharedProcessMessageBuilderImpl(const CefString& name, size_t byte_size);
+  CefSharedProcessMessageBuilderImpl(const CefProcessMessageSMRImpl&) = delete;
+  CefSharedProcessMessageBuilderImpl& operator=(
+      const CefSharedProcessMessageBuilderImpl&) = delete;
+
+  bool IsValid() override;
+  size_t Size() override;
+  void* Memory() override;
+  CefRefPtr<CefProcessMessage> Build() override;
+
+ private:
+  const CefString name_;
+  base::MappedReadOnlyRegion region_;
+
+  IMPLEMENT_REFCOUNTING(CefSharedProcessMessageBuilderImpl);
+};
+
+#endif  // CEF_LIBCEF_COMMON_PROCESS_MESSAGE_SMR_IMPL_H_