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

net/tools/flip_server/spdy_util.h

Index: net/tools/flip_server/spdy_util.h
diff --git a/net/tools/flip_server/spdy_util.h b/net/tools/flip_server/spdy_util.h
index 6f002000089a2a752fcfab7259e67a5e175988d0..a418b3d0322ab7d6910263556b9df6c41007fa82 100644
--- a/net/tools/flip_server/spdy_util.h
+++ b/net/tools/flip_server/spdy_util.h
@@ -13,7 +13,9 @@ namespace net {
 extern bool g_need_to_encode_url;
 
 // Encode the URL.
-std::string EncodeURL(std::string uri, std::string host, std::string method);
+std::string EncodeURL(const std::string& uri,
+                      const std::string& host,
+                      const std::string& method);
 
 }  // namespace net