Add support for request dataType to network data collection

[juliandescottes]

Fixes #748

• Adds another entry point expected to be called from the fetch spec clone network request body (alternatively could be called explicitly from beforeRequestSent for now)
• Extracts a generic algorithm maybe collect network data from maybe collect network response body
• Update maybe collect network response body to call maybe collect network data
• Adds a new algorithm maybe collect network request body called from the beforeRequestSent steps

Otherwise this mostly reuses the logic added for response body collection. Commands do not need to be udpated AFAICT

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Preview | Diff

[sadym-chromium]

I'd propose to use typed arguments for externally used hooks: #1004

Suggested change

	To <dfn export>clone network request body</dfn> given |request|:
	To <dfn export>clone network request body</dfn> given [=/request=] |request|:

[sadym-chromium]

oh, despite the comment, it is not used externally

[juliandescottes]

In theory it should be called from fetch, but yes at the moment it's blocked on whatwg/fetch#1540

Alternatively I can rewrite the PR to trigger this algorithm from the beforeRequestSent event handler if you prefer?

[sadym-chromium]

This comment is not a blocker (I already approved the PR).

I cannot see in the fetch RP any reference to this hook. Maybe this comment meant to underline that this is a part of the WebDriver BiDi before request sent hook, which is invoked by the fetch spec in fetch RP?

[juliandescottes]

Oh no you're right, I haven't updated James' PR. Not quite sure how to do that but I can give it a try.

And in theory, WebDriver BiDi before request sent is a bit too late, because other consumers might have read the stream already, so we really need to clone it as soon as possible.

[juliandescottes]

Thanks for the review! I addressed the suggestions to add types. Let me know if you want to switch to an internal trigger for the algorithm for now. I'll discuss internally to make the fetch PR move.