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_data_writer.h b/quic/core/quic_data_writer.h
index e107fcb..4e6b179 100644
--- a/quic/core/quic_data_writer.h
+++ b/quic/core/quic_data_writer.h
@@ -122,7 +122,7 @@
 
   size_t remaining() const { return capacity_ - length_; }
 
-  QuicString DebugString() const;
+  std::string DebugString() const;
 
  private:
   // Returns the location that the data should be written at, or nullptr if