[streams] Port precise flow control to writable streams (#318)

After https://github.com/whatwg/streams/pull/317, readable streams will have precise flow control. We should add something similar to writable streams. However, the shape of it will likely depend on the resolution of https://github.com/whatwg/streams/issues/316.

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/318