Cleanup: Pass std::string as const reference if possible

net/spdy/spdy_alt_svc_wire_format_test.cc

 // Copyright (c) 2015 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/spdy/spdy_alt_svc_wire_format.h"
 
 #include "base/logging.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/platform_test.h"
 
@@ skipping 245 matching lines @@
   }
 }
 
 // ParseHeaderFieldValue() should return false on a field values truncated
 // before closing quotation mark, without trying to access memory beyond the end
 // of the input.
 TEST(SpdyAltSvcWireFormatTest, ParseTruncatedHeaderFieldValue) {
   SpdyAltSvcWireFormat::AlternativeServiceVector altsvc_vector;
   const char* field_value_array[] = {
       "p=\":137\"", "p=\"foo:137\"", "p%25=\"foo\\\"bar\\\\baz:137\""};
-  for (std::string field_value : field_value_array) {
+  for (const std::string& field_value : field_value_array) {
     for (size_t len = 1; len < field_value.size(); ++len) {
       EXPECT_FALSE(SpdyAltSvcWireFormat::ParseHeaderFieldValue(
           field_value.substr(0, len), &altsvc_vector))
           << len;
     }
   }
 }
 
 // Tests of private methods.
 
@@ skipping 204 matching lines @@
     StringPiece input(invalid_input);
     double probability;
     EXPECT_FALSE(test::SpdyAltSvcWireFormatPeer::ParseProbability(
         input.begin(), input.end(), &probability));
   }
 }
 
 }  // namespace
 
 }  // namespace net