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.

tests/cefclient/browser/client_browser.cc

@@ -11,6 +11,8 @@
 #include "include/cef_file_util.h"
 #include "tests/cefclient/browser/client_prefs.h"
 #include "tests/cefclient/browser/default_client_handler.h"
+#include "tests/cefclient/browser/main_context.h"
+#include "tests/cefclient/browser/root_window_manager.h"
 #include "tests/shared/common/client_switches.h"
 
 namespace client {
@@ -62,6 +64,20 @@ class ClientBrowserDelegate : public ClientAppBrowser::Delegate {
     }
   }
 
+  bool OnAlreadyRunningAppRelaunch(
+      CefRefPtr<ClientAppBrowser> app,
+      CefRefPtr<CefCommandLine> command_line,
+      const CefString& current_directory) override {
+    // Create a new root window based on |command_line|.
+    auto config = std::make_unique<RootWindowConfig>(command_line->Copy());
+
+    MainContext::Get()->GetRootWindowManager()->CreateRootWindow(
+        std::move(config));
+
+    // Relaunch was handled.
+    return true;
+  }
+
   CefRefPtr<CefClient> GetDefaultClient(
       CefRefPtr<ClientAppBrowser> app) override {
     // Default client handler for unmanaged browser windows. Used with the