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_socket_address_coder.h b/quic/core/quic_socket_address_coder.h
index 6ac5d49..e02fc6e 100644
--- a/quic/core/quic_socket_address_coder.h
+++ b/quic/core/quic_socket_address_coder.h
@@ -25,7 +25,7 @@
   QuicSocketAddressCoder& operator=(const QuicSocketAddressCoder&) = delete;
   ~QuicSocketAddressCoder();
 
-  QuicString Encode() const;
+  std::string Encode() const;
 
   bool Decode(const char* data, size_t length);