Resource Timing behavior for HTTP Exchange loading

[irori]

This came up in https://chromium-review.googlesource.com/c/chromium/src/+/967632.

When a HTTP Exchange is loaded from an application/http-exchange+cbor envelope, what the resource timing fields for that load should represent?

cc: @jyasskin @kinu

[jyasskin]

Resource timing is https://w3c.github.io/resource-timing/.

We'll need to specify all of the fields in https://w3c.github.io/resource-timing/#dom-performanceresourcetiming.

If we treat a signed exchange as a redirect, the normal resource timing algorithms will skip most of the data transfer, so I suspect we want to special-case them to cover the envelope instead of its content.