Request standards positions from Mozilla and WebKit

[reillyeon]

Given that we are getting close to launching an Origin Trial in Chromium for parts of the WebNN API it's time to request more formal standards positions from other browser vendors. This doesn't need to be a complete review, given that there are aspects of the API that are still fuzzy, but this is a conversation that should be happening.

[anssiko]

Added to TPAC agenda webmachinelearning/meetings#25 to discuss expectations. Related feedback from Chrome #453 has been helpful for the group. Since that feedback was submitted, both Apple and Mozilla joined the WG that will help form informed positions.