Remove const data member from ByteStream to make it copyable

src/core/lib/transport/byte_stream.h

@@ -72,7 +72,7 @@ class ByteStream : public Orphanable {
       : length_(length), flags_(flags) {}
 
  private:
-  const uint32_t length_;
+  uint32_t length_;
   uint32_t flags_;
 };