[streaming-fetch] The percentage calculator is broken

[ferrybig]

I'm trying to use the fetch download progress code you shared https://twitter.com/umaar/status/917789464658890753/photo/1

Demo: https://sitepen.github.io/javascript-streams-blog-examples/streaming-fetch/

Instead of a normal progress going from 0 to 100%, its going to 25508% (chunk 73)

According to my network tab, the following headers are send:

content-length: 18966
content-encoding: gzip

While the actual file size is 4.8MB

But the code seems to assume the content length is the file length