Add more string_util functions to base namespace.

net/test/url_request/url_request_slow_download_job.cc

Index: net/test/url_request/url_request_slow_download_job.cc
diff --git a/net/test/url_request/url_request_slow_download_job.cc b/net/test/url_request/url_request_slow_download_job.cc
index 2d9049f7d9c79309ed735e524ffe0ff98c3ccf1a..f344feb1c4d8f69f22008c6ef9fee22cf70f0997 100644
--- a/net/test/url_request/url_request_slow_download_job.cc
+++ b/net/test/url_request/url_request_slow_download_job.cc
@@ -273,7 +273,8 @@ void URLRequestSlowDownloadJob::GetResponseInfoConst(
   }
 
   // ParseRawHeaders expects \0 to end each header line.
-  ReplaceSubstringsAfterOffset(&raw_headers, 0, "\n", std::string("\0", 1));
+  base::ReplaceSubstringsAfterOffset(
+      &raw_headers, 0, "\n", base::StringPiece("\0", 1));
   info->headers = new HttpResponseHeaders(raw_headers);
 }