Add SpdyString alias for std::string in net/spdy.

net/spdy/spdy_log_util.h

Index: net/spdy/spdy_log_util.h
diff --git a/net/spdy/spdy_log_util.h b/net/spdy/spdy_log_util.h
index 1c5649193d69fd30da04788184d84e8c8ee0eab4..e2f98e6c55c31d26d1869c9c15cd4dd893806dd9 100644
--- a/net/spdy/spdy_log_util.h
+++ b/net/spdy/spdy_log_util.h
@@ -6,11 +6,11 @@
 #define NET_SPDY_SPDY_LOG_UTIL_H_
 
 #include <memory>
-#include <string>
 
 #include "base/strings/string_piece.h"
 #include "net/base/net_export.h"
 #include "net/log/net_log_capture_mode.h"
+#include "net/spdy/platform/api/spdy_string.h"
 #include "net/spdy/spdy_header_block.h"
 
 namespace base {
@@ -21,9 +21,9 @@ namespace net {
 
 // Given an HTTP/2 GOAWAY frame |debug_data|, returns the elided version
 // according to |capture_mode|.
-NET_EXPORT_PRIVATE std::string ElideGoAwayDebugDataForNetLog(
-    NetLogCaptureMode capture_mode,
-    base::StringPiece debug_data);
+NET_EXPORT_PRIVATE SpdyString
+ElideGoAwayDebugDataForNetLog(NetLogCaptureMode capture_mode,
+                              base::StringPiece debug_data);
 
 // Given a SpdyHeaderBlock, return its base::ListValue representation.
 NET_EXPORT_PRIVATE std::unique_ptr<base::ListValue>