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/core/http/quic_spdy_client_stream.h b/quic/core/http/quic_spdy_client_stream.h
index 8a66740..521914a 100644
--- a/quic/core/http/quic_spdy_client_stream.h
+++ b/quic/core/http/quic_spdy_client_stream.h
@@ -57,7 +57,7 @@
                      bool fin);
 
   // Returns the response data.
-  const QuicString& data() { return data_; }
+  const std::string& data() { return data_; }
 
   // Returns whatever headers have been received for this stream.
   const spdy::SpdyHeaderBlock& response_headers() { return response_headers_; }
@@ -83,7 +83,7 @@
   // The parsed content-length, or -1 if none is specified.
   int64_t content_length_;
   int response_code_;
-  QuicString data_;
+  std::string data_;
   size_t header_bytes_read_;
   size_t header_bytes_written_;