Ask how each feature was proposed to a working group

[jyasskin]

Is your feature request related to a problem? Please describe.

https://www.chromium.org/blink/launching-features/#new-feature-prepare-to-ship says "If your specification is still in an incubation venue and not a working group, propose that the feature migrate to a working group." But it's easy to forget this since there's no matching field in Chrome Status.

Describe the solution you'd like

First, #3390 proposes to add the knowledge of which WG is expected to eventually adopt a feature. Then as part of the I2S, we should ask for a statement or link to how the proponents asked to migrate the feature.

cc/ @past