Replace QuicString with std::string, pass 1

This replaces QuicString with std::string in all of the "QUIC proper".  I will
delete QuicString once all code using it is gone.

gfe-relnote: n/a (no functional change)
PiperOrigin-RevId: 237872023
Change-Id: I82de62c9855516b15039734d05155917e68ff4ee
diff --git a/quic/tools/quic_backend_response.h b/quic/tools/quic_backend_response.h
index cd052c8..4ef0fdc 100644
--- a/quic/tools/quic_backend_response.h
+++ b/quic/tools/quic_backend_response.h
@@ -20,13 +20,13 @@
     ServerPushInfo(QuicUrl request_url,
                    spdy::SpdyHeaderBlock headers,
                    spdy::SpdyPriority priority,
-                   QuicString body);
+                   std::string body);
     ServerPushInfo(const ServerPushInfo& other);
 
     QuicUrl request_url;
     spdy::SpdyHeaderBlock headers;
     spdy::SpdyPriority priority;
-    QuicString body;
+    std::string body;
   };
 
   enum SpecialResponseType {
@@ -75,7 +75,7 @@
   SpecialResponseType response_type_;
   spdy::SpdyHeaderBlock headers_;
   spdy::SpdyHeaderBlock trailers_;
-  QuicString body_;
+  std::string body_;
   uint16_t stop_sending_code_;
 };