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

net/spdy/spdy_prefixed_buffer_reader_test.cc

Index: net/spdy/spdy_prefixed_buffer_reader_test.cc
diff --git a/net/spdy/spdy_prefixed_buffer_reader_test.cc b/net/spdy/spdy_prefixed_buffer_reader_test.cc
index e3a6a6552dc84b3004db8af57cf4f6b1dd321216..406bab71618cefe3198b0caa846b0e4acd46bd06 100644
--- a/net/spdy/spdy_prefixed_buffer_reader_test.cc
+++ b/net/spdy/spdy_prefixed_buffer_reader_test.cc
@@ -16,7 +16,8 @@ using testing::ElementsAreArray;
 
 class SpdyPrefixedBufferReaderTest : 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(),