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.cc

@@ -0,0 +1,43 @@
+// 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.
+
+#include "libcef/browser/chrome/chrome_startup_browser_creator.h"
+
+#include <tuple>
+
+#include "libcef/browser/browser_context.h"
+#include "libcef/common/app_manager.h"
+#include "libcef/common/command_line_impl.h"
+
+#include "chrome/browser/ui/startup/startup_browser_creator.h"
+
+namespace startup_browser_creator {
+
+namespace {
+
+bool ProcessCommandLineCallback(const base::CommandLine& command_line,
+                                const base::FilePath& cur_dir) {
+  bool handled = false;
+
+  if (auto app = CefAppManager::Get()->GetApplication()) {
+    if (auto handler = app->GetBrowserProcessHandler()) {
+      CefRefPtr<CefCommandLineImpl> commandLinePtr(
+          new CefCommandLineImpl(command_line));
+      handled = handler->OnAlreadyRunningAppRelaunch(commandLinePtr.get(),
+                                                     cur_dir.value());
+      std::ignore = commandLinePtr->Detach(nullptr);
+    }
+  }
+
+  return handled;
+}
+
+}  // namespace
+
+void RegisterProcessCommandLineCallback() {
+  StartupBrowserCreator::RegisterProcessCommandLineCallback(
+      base::BindRepeating(&ProcessCommandLineCallback));
+}
+
+}  // namespace startup_browser_creator