[css-highlight-api] highlightsFromPoint(): Inconsistency between spec and test regarding shadow DOM

[jnjaeschke]

There is a test which deals with a live range that starts and ends in light DOM, but has a shadow DOM part inside.

The test asserts that highlightsFromPoint() does not return anything when being called over the shadow DOM part.

However, the spec says that highlightsFromPoint() should return a Highlight if any of the DOMRects returned by getClientRects() of any range of the highlight intersects the point.

Both Gecko and Chrome also return DOMRects for the shadow DOM part, therefore the implementation would include the shadow DOM parts as well.

cc @ffiori @emilio @smaug----