Way to explicitly register share targets

[mgiuca]

We may wish to provide a method for websites to explicitly request to prompt the user for handler registration. This would be limited to prompting to register the share_target already declared in the manifest (not dynamically creating share targets). For now, we have omitted such a method from the design to keep control in the hands of user agents. It is easier to add such a method later than remove it.

[kenchris]

Seems like a v2 feature

[mgiuca]

Yep.