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

net/http/http_cache_unittest.cc

Index: net/http/http_cache_unittest.cc
diff --git a/net/http/http_cache_unittest.cc b/net/http/http_cache_unittest.cc
index 12a849c027ce663887f893036a4bc519b01f337b..5992b3ba075964224862e36a248e26a76f0ed6ef 100644
--- a/net/http/http_cache_unittest.cc
+++ b/net/http/http_cache_unittest.cc
@@ -463,7 +463,7 @@ const char kFullRangeData[] =
 
 // Verifies the response headers (|response|) match a partial content
 // response for the range starting at |start| and ending at |end|.
-void Verify206Response(std::string response, int start, int end) {
+void Verify206Response(const std::string& response, int start, int end) {
   std::string raw_headers(
       HttpUtil::AssembleRawHeaders(response.data(), response.size()));
   scoped_refptr<HttpResponseHeaders> headers(