Fix crasher if onBeforeSendHeaders contains no new headers

chrome/browser/extensions/extension_webrequest_api_helpers.cc

 // Copyright (c) 2011 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 "chrome/browser/extensions/extension_webrequest_api_helpers.h"
 
 #include "base/string_util.h"
 #include "base/values.h"
 #include "chrome/browser/extensions/extension_webrequest_api.h"
 #include "chrome/common/url_constants.h"
@@ skipping 122 matching lines @@
 EventResponseDelta* CalculateOnBeforeSendHeadersDelta(
     const std::string& extension_id,
     const base::Time& extension_install_time,
     bool cancel,
     net::HttpRequestHeaders* old_headers,
     net::HttpRequestHeaders* new_headers) {
   EventResponseDelta* result =
       new EventResponseDelta(extension_id, extension_install_time);
   result->cancel = cancel;
 
-  // Find deleted headers.
-  {
-    net::HttpRequestHeaders::Iterator i(*old_headers);
-    while (i.GetNext()) {
-      if (!new_headers->HasHeader(i.name())) {
-        result->deleted_request_headers.push_back(i.name());
+  // The event listener might not have passed any new headers if he
+  // just wanted to cancel the request.
+  if (new_headers) {
+    // Find deleted headers.
+    {
+      net::HttpRequestHeaders::Iterator i(*old_headers);
+      while (i.GetNext()) {
+        if (!new_headers->HasHeader(i.name())) {
+          result->deleted_request_headers.push_back(i.name());
+        }
+      }
+    }
+
+    // Find modified headers.
+    {
+      net::HttpRequestHeaders::Iterator i(*new_headers);
+      while (i.GetNext()) {
+        std::string value;
+        if (!old_headers->GetHeader(i.name(), &value) || i.value() != value) {
+          result->modified_request_headers.SetHeader(i.name(), i.value());
+        }
       }
     }
   }
-
-  // Find modified headers.
-  {
-    net::HttpRequestHeaders::Iterator i(*new_headers);
-    while (i.GetNext()) {
-      std::string value;
-      if (!old_headers->GetHeader(i.name(), &value) || i.value() != value) {
-        result->modified_request_headers.SetHeader(i.name(), i.value());
-      }
-    }
-  }
   return result;
 }
 
 EventResponseDelta* CalculateOnHeadersReceivedDelta(
     const std::string& extension_id,
     const base::Time& extension_install_time,
     bool cancel,
     net::HttpResponseHeaders* old_response_headers,
     ResponseHeaders* new_response_headers) {
   EventResponseDelta* result =
@@ skipping 361 matching lines @@
               new NetLogExtensionIdParameter((*delta)->extension_id)));
       event_log_entries->push_back(log_entry);
       *auth_credentials = *(*delta)->auth_credentials;
       credentials_set = true;
     }
   }
   return credentials_set;
 }
 
 }  // namespace extension_webrequest_api_helpers