Remove Event.relatedTargetScoped and update event path calculation for relatedTarget

See the context: https://github.com/w3c/webcomponents/issues/486

The spec change:
- https://github.com/w3c/webcomponents/commit/f113e26bfa999bac730c65ea7418fe787500313c
- https://github.com/w3c/webcomponents/commit/b1433e5f3a013ca825f02fb8f7b0e9d6217c0125

BUG=531990

Review-Url: https://codereview.chromium.org/2030513002
Cr-Commit-Position: refs/heads/master@{#397329}
31 files changed