Update Web Share and Web Share Target positions

[marcoscaceres]

closes #27
closes #176

[marcoscaceres]

@dbaron, ping. Web Share itself is ready to land behind a pref:
https://phabricator.services.mozilla.com/D44598

[dbaron]

Sorry for the delay @marcoscaceres -- I'm back from vacation now.

One concern about this PR is that it updates the position on Web Share to important without any discussion of that change in #27. Could you discuss that particular change there?

It might also be worth writing a little bit of text in the mozPositionDetail explaining why these specifications are worth prototyping or important.

[marcoscaceres]

I'll defer to the product owners to provide positions, as they are in charge of adding these to Firefox.

[marcoscaceres]

One concern about this PR is that it updates the position on Web Share to important without any discussion of that change in #27. Could you discuss that particular change there?

@dbaron, I added some thoughts of why I think it's important - hopefully other folks will chime in. The feature is already in Fenix, so hopefully we can reach a position before it ships. I imagine the Fenix team will send an "intent to ship" at some point.

#27 (comment)