Add setHandlingEvent method the headless_application for Mac

headless/lib/browser/headless_shell_application_mac.mm

 // Copyright 2017 The Chromium 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 "headless/lib/browser/headless_shell_application_mac.h"
 
 #include "base/auto_reset.h"
 
 @implementation HeadlessShellCrApplication
 
 - (BOOL)isHandlingSendEvent {
   // Since headless mode is non-interactive, always return false.
   return false;
 }
 
+// This method is required to allow handling some input events on Mac OS.
+- (void)setHandlingSendEvent:(BOOL)handlingSendEvent {
+}
+
 @end