Add callbacks for unresponsive render process (fixes #3661)

The client can optionally wait or terminate the render process.

Expose process exit codes via OnRenderProcessTerminated and
CefGetExitCode (fixes #2126).

cefclient: Add a new https://tests/hang page for testing hang behavior.

cefclient: Move message and resource handling to a new BaseClientHandler
class to support loading of test pages in default Chrome UI windows.

libcef_dll/cpptoc/unresponsive_process_callback_cpptoc.h

@@ -0,0 +1,38 @@
+// Copyright (c) 2024 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.
+//
+// ---------------------------------------------------------------------------
+//
+// This file was generated by the CEF translator tool. If making changes by
+// hand only do so within the body of existing method and function
+// implementations. See the translator.README.txt file in the tools directory
+// for more information.
+//
+// $hash=1816748407afd5599d8582de51b3289d1ea24a33$
+//
+
+#ifndef CEF_LIBCEF_DLL_CPPTOC_UNRESPONSIVE_PROCESS_CALLBACK_CPPTOC_H_
+#define CEF_LIBCEF_DLL_CPPTOC_UNRESPONSIVE_PROCESS_CALLBACK_CPPTOC_H_
+#pragma once
+
+#if !defined(BUILDING_CEF_SHARED)
+#error This file can be included DLL-side only
+#endif
+
+#include "include/capi/cef_unresponsive_process_callback_capi.h"
+#include "include/cef_unresponsive_process_callback.h"
+#include "libcef_dll/cpptoc/cpptoc_ref_counted.h"
+
+// Wrap a C++ class with a C structure.
+// This class may be instantiated and accessed DLL-side only.
+class CefUnresponsiveProcessCallbackCppToC
+    : public CefCppToCRefCounted<CefUnresponsiveProcessCallbackCppToC,
+                                 CefUnresponsiveProcessCallback,
+                                 cef_unresponsive_process_callback_t> {
+ public:
+  CefUnresponsiveProcessCallbackCppToC();
+  virtual ~CefUnresponsiveProcessCallbackCppToC();
+};
+
+#endif  // CEF_LIBCEF_DLL_CPPTOC_UNRESPONSIVE_PROCESS_CALLBACK_CPPTOC_H_