Clear event's target and relatedTarget when dispatch ends at a shadow root

[zqzhang]

• [ ] https://github.com/whatwg/dom/commit/9ada239e14fe7964577276ef9d8972069e665247 Clear event's target and relatedTarget when dispatch ends at a shadow root

[zqzhang]

http://w3c-test.org/shadow-dom/event-post-dispatch.html

For this test file, Chrome 61.0.3163.100 returns 5 failed cases, while Firefox Nightly 58.0a1 (2017-10-13) failed to run it.

[zqzhang]

Anyway, this will be addressed in next release.