chrome: Add callback for already running app relaunch (fixes #3609)

Adds a new CefBrowserProcessHandler::OnAlreadyRunningAppRelaunch
callback for when an already running app is relaunched with the
same CefSettings.root_cache_path.

Client apps should check the CefInitialize() return value for early
exit of the relaunch source process.

libcef/browser/chrome/chrome_startup_browser_creator.h

@@ -0,0 +1,16 @@
+// Copyright (c) 2023 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_CHROME_CHROME_STARTUP_BROWSER_CREATOR_H_
+#define CEF_LIBCEF_BROWSER_CHROME_CHROME_STARTUP_BROWSER_CREATOR_H_
+#pragma once
+
+namespace startup_browser_creator {
+
+// Register the process launch command line callback.
+void RegisterProcessCommandLineCallback();
+
+}  // namespace startup_browser_creator
+
+#endif  // CEF_LIBCEF_BROWSER_CHROME_CHROME_STARTUP_BROWSER_CREATOR_H_