Rewrite std::string("") to std::string(), Linux edition.

net/url_request/url_request_throttler_test_support.cc

 // Copyright (c) 2012 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 "net/url_request/url_request_throttler_test_support.h"
 
 #include "net/url_request/url_request_throttler_entry.h"
 
 namespace net {
 
@@ skipping 30 matching lines @@
 }
 
 std::string MockURLRequestThrottlerHeaderAdapter::GetNormalizedValue(
     const std::string& key) const {
   if (key ==
       URLRequestThrottlerEntry::kExponentialThrottlingHeader &&
       !fake_opt_out_value_.empty()) {
     return fake_opt_out_value_;
   }
 
-  return "";
+  return std::string();
 }
 
 int MockURLRequestThrottlerHeaderAdapter::GetResponseCode() const {
   return fake_response_code_;
 }
 
 }  // namespace net