Feature-Policy and Workers

[bakulf]

FeaturePolicy should be supported in Workers and SharedWorkers.
Policy interface should be also exposed via DedicatedWorkerGlobalScope and SharedWorkerGlobalScope.

[clelland]

#174 is an attempt to define support for workers, but may be incomplete.

• Doesn't allow for a Feature-Policy HTTP header to be sent with worker scripts
• SharedWorkers are still problematic -- I think that that the integration in that PR would mean that a shared worker gets the policy of the first document which creates it, which is racy.

[martinthomson]

In review at Mozilla, we identified shared and service workers as serious problems. Having a policy subject to the whims of races is not good. This suggests that this needs more thought.

[rsolomakhin]

cc @rsolomakhin

[oyiptong]

cc @oyiptong