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_decoder_stream_sender.cc b/quic/core/qpack/qpack_decoder_stream_sender.cc
index 141b64a..cdb3b35 100644
--- a/quic/core/qpack/qpack_decoder_stream_sender.cc
+++ b/quic/core/qpack/qpack_decoder_stream_sender.cc
@@ -23,7 +23,7 @@
 
   instruction_encoder_.Encode(InsertCountIncrementInstruction());
 
-  QuicString output;
+  std::string output;
 
   instruction_encoder_.Next(std::numeric_limits<size_t>::max(), &output);
   DCHECK(!instruction_encoder_.HasNext());
@@ -37,7 +37,7 @@
 
   instruction_encoder_.Encode(HeaderAcknowledgementInstruction());
 
-  QuicString output;
+  std::string output;
 
   instruction_encoder_.Next(std::numeric_limits<size_t>::max(), &output);
   DCHECK(!instruction_encoder_.HasNext());
@@ -50,7 +50,7 @@
 
   instruction_encoder_.Encode(StreamCancellationInstruction());
 
-  QuicString output;
+  std::string output;
 
   instruction_encoder_.Next(std::numeric_limits<size_t>::max(), &output);
   DCHECK(!instruction_encoder_.HasNext());