message: Fix compilation error on 32-bit architectures introduced in r350511
  The issue was a narrowing conversion when converting from uint64_t to a
  size_t.
diff: @@ -1096,7 +1096,7 @@ public:
  void Checksum(llvm::SmallVectorImpl<uint8_t> &dest, uint64_t max_data = 0);
  llvm::ArrayRef<uint8_t> GetData() const {
  - return {GetDataStart(), GetByteSize()};
  + return {GetDataStart(), size_t(GetByteSize())};
  }
  protected:
  