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/offline/qpack_offline_decoder.cc b/quic/core/qpack/offline/qpack_offline_decoder.cc
index 2b02edd..137ee7c 100644
--- a/quic/core/qpack/offline/qpack_offline_decoder.cc
+++ b/quic/core/qpack/offline/qpack_offline_decoder.cc
@@ -107,7 +107,7 @@
     QuicStringPiece input_filename) {
   // Store data in |input_data_storage|; use a QuicStringPiece to efficiently
   // keep track of remaining portion yet to be decoded.
-  QuicString input_data_storage;
+  std::string input_data_storage;
   ReadFileContents(input_filename, &input_data_storage);
   QuicStringPiece input_data(input_data_storage);
 
@@ -166,7 +166,7 @@
     QuicStringPiece expected_headers_filename) {
   // Store data in |expected_headers_data_storage|; use a QuicStringPiece to
   // efficiently keep track of remaining portion yet to be decoded.
-  QuicString expected_headers_data_storage;
+  std::string expected_headers_data_storage;
   ReadFileContents(expected_headers_filename, &expected_headers_data_storage);
   QuicStringPiece expected_headers_data(expected_headers_data_storage);