PDF.js should abort fetch requests during teardown

[johnhok]

Steps to reproduce the problem:

1. Fetch a document with streaming enabled.
2. Destroy the document loading task
3. Observe that the fetch request is not aborted.

What is the expected behavior?
Fetch request should be aborted when cancel is called on PDFFetchStreamReader and PDFFetchStreamRangeReader.

Context
It seems when fetch was implemented (#6126 (comment)) abort on fetch was still in flux.

It seems now that has settled and chromium has also implemented aborting fetch requests in 66 (https://bugs.chromium.org/p/chromium/issues/detail?id=750599)

[mukulmishra18]

Sounds reasonable to me. I think it is good idea to abort fetch in PDFFetchStream when document destroy.

I think all we need is to have a property something like this.controller = new AbortController() in PDFFetchStreamReader and PDFFetchStreamRangeReader and pass it to the fetch(url, { this.controller.signal, ... }) and call this.controller.abort() in cancel method.

@yurydelendik @Rob--W What you think about that?

[Rob--W]

Sounds good to me. Make sure to use feature detection before using AbortController.

[mukulmishra18]

Make sure to use feature detection before using AbortController.

Okay, sure. I will try to create a PR for this tonight.