browser.setDownloadBehavior

[sadym-chromium]

In puppeteer, there is a way to deny all the downloads, or allow for downloading to a custom folder. This PR is intended to provide a way to customize the download behavior per user context or globally.

1. Should be followed up with HTML PR invoking new hooks.
2. Add a new hook "WebDriver BiDi download will begin" which is intended to replace "WebDriver BiDi download started".
3. "WebDriver BiDi download will begin" returns "download behavior struct", which instructs the user agent how to handle downloads.

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[juliandescottes]

I think this is worth discussing with the working group before doing an in-depth review.

I can see most automation libraries provide ways to set a download path, so that sounds like a useful feature. Libraries could already monitor downloads and move files manually to achieve the same effect, but still having it built in sounds more practical. And there are already preferences to drive this in most browsers anyway...

Regarding the API itself, I can see CDP has this on the Browser domain, but it also has the download events there, whereas for us they are on BrowsingContext. I would have expected this on BrowsingContext, but interested to know what others think.

[sadym-chromium]

I can see most automation libraries provide ways to set a download path, so that sounds like a useful feature. Libraries could already monitor downloads and move files manually to achieve the same effect, but still having it built in sounds more practical. And there are already preferences to drive this in most browsers anyway...

Testing libraries are not guaranteed to have access to the filesystem, so I would leave this logic to the browser.

Regarding the API itself, I can see CDP has this on the Browser domain, but it also has the download events there, whereas for us they are on BrowsingContext. I would have expected this on BrowsingContext, but interested to know what others think.

The Browser module was chosen as implementation-wise in CDP we cannot set the download behavior per browsing context, only per user context. So having this method in BrowsingContext module and not being able to configure per browsing context seems a bit odd.

[sadym-chromium]

@juliandescottes PTAL