Add CrAppProtocol for headless on Mac

headless/lib/browser/headless_browser_main_parts_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_browser_main_parts.h"
+
+#import <Cocoa/Cocoa.h>
+
+#include "headless/lib/browser/headless_shell_application_mac.h"
+
+namespace headless {
+
+void HeadlessBrowserMainParts::PreMainMessageLoopStart() {
+  // Force the NSApplication subclass to be used.
+  [HeadlessShellCrApplication sharedApplication];
+}
+
+}  // namespace headless