Re: [webcomponents] caretPositionFromPoint, elementFromPoint, and elementsFromPoint should respect shadow boundary (#313)

As far as I can tell, only the definition of `elementFromPoint` says that.  `caretPositionFromPoint` and `elementsFromPoint` are entirely missing their definitions.

Also, the current definition of `elementFromPoint` refers to an undefined term "hit testing".  I think it's best to rely on the corresponding functions on `Document` instead: https://drafts.csswg.org/cssom-view/#extensions-to-the-document-interface

In addition, the CSSOM also need to respect the shadow boundary and run the retargeting algorithm as well since we wouldn't be returning "associated elements" inside shadow boundaries.  At that point, since CSSOM will depend on this shadow DOM API spec, they might as well as add these functions to both `Document` and `ShadowRoot`.

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https://github.com/w3c/webcomponents/issues/313#issuecomment-137754436