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/qpack/qpack_progressive_encoder.cc b/quic/core/qpack/qpack_progressive_encoder.cc
index 41433ff..497465b 100644
--- a/quic/core/qpack/qpack_progressive_encoder.cc
+++ b/quic/core/qpack/qpack_progressive_encoder.cc
@@ -37,7 +37,7 @@
 }
 
 void QpackProgressiveEncoder::Next(size_t max_encoded_bytes,
-                                   QuicString* output) {
+                                   std::string* output) {
   DCHECK_NE(0u, max_encoded_bytes);
   DCHECK(HasNext());