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

net/spdy/spdy_pinnable_buffer_piece_test.cc

Index: net/spdy/spdy_pinnable_buffer_piece_test.cc
diff --git a/net/spdy/spdy_pinnable_buffer_piece_test.cc b/net/spdy/spdy_pinnable_buffer_piece_test.cc
index 456e4a86defe745a7ee3471b97c4c90a48b0ab7a..3101f5ea239ea9220b9dbb0039c7318a715af71b 100644
--- a/net/spdy/spdy_pinnable_buffer_piece_test.cc
+++ b/net/spdy/spdy_pinnable_buffer_piece_test.cc
@@ -15,7 +15,8 @@ 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(),