[w3c/webcomponents] Shadow tree encapsulation and Element Timing (#816)

We're incubating the [ElementTiming](https://github.com/WICG/element-timing) API which exposes rendering timings of elements of a website (namely, image and text content). I filed internally https://github.com/WICG/element-timing/issues/3 to attempt to clarify what the interaction with shadow elements should be (in particular regarding what we can expose from the shadow tree). I realize that an issue there will not be noticed by relevant folks so I figure I can get more attention by filing an issue here.

I understand that there is a desire to have some encapsulation principles. Where are those principles stated? I'd be interested to understand the motivations of them. In this particular case, I'm interested in understanding if/why it would be desirable to hide important performance information from developers when they use shadow trees. I stated some options on how to handle in the other issue, restating here:

* Do no special handling: expose entries, with all the attributes set.
* Expose PerformanceElementTiming entries but with some attributes such as `element` set to null.
* Expose PerformanceElementTiming entries, but setting the `element` attribute value to the shadow host / shadow root.
* Do not expose entries at all.
* Make it opt-in so that a developer must set a bit in order to get entries from shadow DOM.

CC people on the other issue @hayatoito @annevk 

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