Change the preferences to ship WritableStreams and pipeTo

Why separate flags for them, btw?

Backed out for causing wpt failures complaining about pipe.

Backout link: https://hg.mozilla.org/integration/autoland/rev/ca2ce6656e6ba746263a0828b8cac414d3392de5

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=653e9dc63dd5917a7b29878791eac08931454f11&selectedTaskRun=Xf32fOP7RpGR4YrB55q1ww.0

Failure log:
https://treeherder.mozilla.org/logviewer?job_id=371591842&repo=autoland&lineNumber=2160
https://treeherder.mozilla.org/logviewer?job_id=371595818&repo=autoland&lineNumber=7220
https://treeherder.mozilla.org/logviewer?job_id=371595811&repo=autoland&lineNumber=3088

Appears I missed updating web-platform/meta/fetch/api/response/response-stream-disturbed-by-pipe.any.js.ini; I've got a try push re-evaluating that, and will re-land tomorrow assuming it's good.

(In reply to Kagami :saschanaz from comment #1)

Why separate flags for them, btw?

Whoops. Missed this. Essentially because they're two different features that can be exposed separately, even if we think that's not the right approach. Could have done it differently, but not worth changing now.

https://hg.mozilla.org/mozilla-central/rev/b58cc8e5384a

FF100 docs work for this can be tracked in https://github.com/mdn/content/issues/14408#issuecomment-1085329853

FYI This is mostly complete - the docs already existed (though needed a tidy up to remove experimental tagging). The main work was browser compatibility update and update to the release note page.