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/quic_crypto_client_stream.h b/quic/core/quic_crypto_client_stream.h
index da149fe..ad6518b 100644
--- a/quic/core/quic_crypto_client_stream.h
+++ b/quic/core/quic_crypto_client_stream.h
@@ -90,7 +90,7 @@
     // ChannelIDSource operated asynchronously. Intended for testing.
     virtual bool WasChannelIDSourceCallbackRun() const = 0;
 
-    virtual QuicString chlo_hash() const = 0;
+    virtual std::string chlo_hash() const = 0;
 
     // Returns true once any encrypter (initial/0RTT or final/1RTT) has been set
     // for the connection.
@@ -156,7 +156,7 @@
   // ChannelIDSource operated asynchronously. Intended for testing.
   bool WasChannelIDSourceCallbackRun() const;
 
-  QuicString chlo_hash() const;
+  std::string chlo_hash() const;
 
  protected:
   void set_handshaker(std::unique_ptr<HandshakerDelegate> handshaker) {