Always return false on IsHandlingSendingEvent

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 {
-  // The CrAppProtocol must return true if [NSApplication sendEvent:] is
-  // currently on the stack. seee |CrAppProtocol| in |MessagePumpMac|.
-  return true;
+  // Since headless mode is non-interactive, always return false.
+  return false;
 }
 
 @end