[webcomponents] Event path trimming should only happen for mouseover and mouseout (#330)

It's very strange for the [event path trimming algorithm](https://w3c.github.io/webcomponents/spec/shadow/#dfn-event-path-trimming-algorithm) to apply trusted events with `relatedTarget`.

Why don't we just do this for all `mouseover` and `mouseout` events and nothing else.  Alternatively, we can add a new constructor option to MouseEvent's init dictionary like `stopsAtLowestCommonShadowRoot` to explicitly set this behavior so that authors can adjust it.  If we did the latter, we'd just make UAs create MouseEvents with that flag set to `true` when firing trusted events.

@travisleithead @annevk: any opinions?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/330