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

net/spdy/spdy_pinnable_buffer_piece_test.cc

 // Copyright 2014 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_pinnable_buffer_piece.h"
 
 #include "net/spdy/spdy_prefixed_buffer_reader.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace net {
 
 namespace test {
 
 using base::StringPiece;
 
 class SpdyPinnableBufferPieceTest : public ::testing::Test {
  protected:
-  SpdyPrefixedBufferReader Build(std::string prefix, std::string suffix) {
+  SpdyPrefixedBufferReader Build(const std::string& prefix,
+                                 const std::string& suffix) {
     prefix_ = prefix;
     suffix_ = suffix;
     return SpdyPrefixedBufferReader(prefix_.data(), prefix_.length(),
                                     suffix_.data(), suffix_.length());
   }
   std::string prefix_, suffix_;
 };
 
 TEST_F(SpdyPinnableBufferPieceTest, Pin) {
   SpdyPrefixedBufferReader reader = Build("foobar", "");
@@ skipping 41 matching lines @@
   SpdyPinnableBufferPiece empty;
   piece2.Swap(&empty);
 
   EXPECT_EQ(StringPiece(""), piece2);
   EXPECT_FALSE(piece2.IsPinned());
 }
 
 }  // namespace test
 
 }  // namespace net