Similar to bug 1723145, which removed the usage of OS.File for Marionette, we have to remove it from our partial CDP implementation:

https://searchfox.org/mozilla-central/search?q=OS.File&path=remote%2F&case=false&regexp=false

All the found instances need to be replaced by IOUtils and PathUtils. See the following commit to get an idea what needs to be done:

https://hg.mozilla.org/mozilla-central/rev/621e47647e8b

Hey! I think I can make the required changes.
But I am not able to figure out where to make changes in
https://searchfox.org/mozilla-central/source/remote/cdp/StreamRegistry.jsm#54
Can you please guide me through this?
Thank you

Kirti, I'm sorry for replying that late but due to some reason I haven't seen your comment. In the case you are still around and also still be interested to work on this bug please let me know. I'm more than happy to provide the necessary details to get you started. Sorry again.

This patch replaces the internals of the StreamRegistry et al to use
nsIStorageStream as the stream type. A storage stream is not directly readable,
so registry returns a stream and read handle (as a nsIBinaryInputStream) for
reading.

Backed out for causing multiple failures

Backout link: https://hg.mozilla.org/integration/autoland/rev/2de5f759dac6e348cd5b8a3e20d96fa99bd42815

Push with failures

Failure log mochitest

Failure log xpc

Barret, I had a look at bug 1603739 and I can no longer reproduce the underlying issue and closed it. As such feel free to also remove the whole block for waiting until the PDF file has been completely written to disk. Thanks.

Backed out for causing xpc failures in remote/cdp/test/xpcshell/test_StreamRegistry.js

Backout link: https://hg.mozilla.org/integration/autoland/rev/5b8107f660cf44deb159a9a0c9d9f1086e8db714

Push with failures

[Failure log xpc] (https://treeherder.mozilla.org/logviewer?job_id=402079526&repo=autoland&lineNumber=2489)

Failure log remote

https://hg.mozilla.org/mozilla-central/rev/3523d141513d