Add download id to download events.

[lutien]

Closes #1096.

Motivation:

• Make it possible to map downloadWillBegin and downloadEnd events in case the navigation id is missing.
• Unblock a possibility of commands to manipulate downloads, e.g., canceling downloads.

The PR in the HTML spec to pass downloadResponse to events: whatwg/html#12426.

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[css-meeting-bot]

The Browser Testing and Tools Working Group just discussed Add download ID field to download events.

The full IRC log of that discussion

<jgraham> Topic: Add download ID field to download events
<jgraham> github: https://github.com//pull/1117
<jgraham> RRSAgent: make logs public
<RRSAgent> I have made the request, jgraham
<tidoust> sasha: The load event only has navigation ID, which is fine for most cases. In the case when it's not present, it makes it impossible to link start/end loading events.
<tidoust> s/load/download
<tidoust> ... I created a PR for that which shouldn't be controversial. Comments welcome.
<orkon> q+
<jgraham> ack next
<tidoust> ack orkon
<tidoust> orkon: I think it looks fine and that we should have a download ID. I'll take another look next week.