Deliver errors from devtools API calls to callbacks

headless/lib/browser/devtools_api/domain_cc.template

 // This file is generated
 
 // Copyright 2016 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/public/devtools/domains/{{domain.domain | camelcase_to_hacker_style}}.h"
 
 #include "base/bind.h"
 
@@ skipping 84 matching lines @@
 
 {# Generate response handlers for commands that need them. #}
 {% for command in domain.commands %}
   {% if not "returns" in command %}{% continue %}{% endif %}
   {% set method_name = command.name | to_title_case %}
 
 // static
 void Domain::Handle{{method_name}}Response(base::Callback<void(std::unique_ptr<{{method_name}}Result>)> callback, const base::Value& response) {
   if (callback.is_null())
     return;
+  // This is an error response.
+  if (response.IsType(base::Value::TYPE_NULL)) {
+    callback.Run(nullptr);
+    return;
+  }
   ErrorReporter errors;
   std::unique_ptr<{{method_name}}Result> result = {{method_name}}Result::Parse(response, &errors);
   DCHECK(!errors.HasErrors());
   callback.Run(std::move(result));
 }
 {% endfor %}
 {% if "events" in domain %}
   {% for event in domain.events %}
 
 {# Generate dispatchers which call registered observers for an event. #}
@@ skipping 26 matching lines @@
 }
 
 void ExperimentalDomain::RemoveObserver(ExperimentalObserver* observer) {
   observers_.RemoveObserver(observer);
 }
   {% endif %}
 
 }  // namespace {{domain.domain | camelcase_to_hacker_style}}
 
 } // namespace headless