Make some ShadowRoot attributes mutable

[mfreed7]

What is the issue with the DOM Standard?

See whatwg/html#10107 for context.

But some fields of ShadowRoot feel like they could easily be made mutable:

• delegatesFocus
• clonable
• serializable (hopefully coming soon)

Others might be more difficult or not desirable:

• mode
• slotAssignment
• host

Should the first set be made mutable?

Also from this comment, it would make sense if mutating any such attributes also set the declarative flag to false.