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.

include/cef_browser_process_handler.h

@@ -97,6 +97,32 @@ class CefBrowserProcessHandler : public virtual CefBaseRefCounted {
   virtual void OnBeforeChildProcessLaunch(
       CefRefPtr<CefCommandLine> command_line) {}
 
+  ///
+  /// Implement this method to provide app-specific behavior when an already
+  /// running app is relaunched with the same CefSettings.root_cache_path value.
+  /// For example, activate an existing app window or create a new app window.
+  /// |command_line| will be read-only. Do not keep a reference to
+  /// |command_line| outside of this method. Return true if the relaunch is
+  /// handled or false for default relaunch behavior. Default behavior will
+  /// create a new default styled Chrome window.
+  ///
+  /// To avoid cache corruption only a single app instance is allowed to run for
+  /// a given CefSettings.root_cache_path value. On relaunch the app checks a
+  /// process singleton lock and then forwards the new launch arguments to the
+  /// already running app process before exiting early. Client apps should
+  /// therefore check the CefInitialize() return value for early exit before
+  /// proceeding.
+  ///
+  /// This method will be called on the browser process UI thread. Currently
+  /// only used with the chrome runtime.
+  ///
+  /*--cef(optional_param=current_directory)--*/
+  virtual bool OnAlreadyRunningAppRelaunch(
+      CefRefPtr<CefCommandLine> command_line,
+      const CefString& current_directory) {
+    return false;
+  }
+
   ///
   /// Called from any thread when work has been scheduled for the browser
   /// process main (UI) thread. This callback is used in combination with