Add attribute to <template shadowroot> to disable upgrades

[justinfagnani]

In order to make scoped custom element registries and declarative shadow DOM work together we need a way to defer upgrading of elements in some shadow roots until their custom element registry has been set.

An attribute can tell the browser to either not upgrade the contained elements, or possibly to create an empty registry which would have the same effect.

<my-element>
  <template shadowroot scoped-custom-elements>
</my-element>

Then when the definition of my-element loads, it can set the registry, in conjunction with #3, to trigger upgrades.

This should be specified here, to make this proposal layer on top of declarative shadow DOM.

cc @mfreed7