Change Event.deepPath to a method

In order to make clear that computation is performed, Event.deepPath has been changed to a method Event.deepPath(), that returns sequence<EventTarget>.
See the discussion here: https://github.com/w3c/webcomponents/issues/361
Described in the spec here : http://w3c.github.io/webcomponents/spec/shadow/#widl-Event-deepPath-sequence-EventTarget
See the spec change here: https://github.com/w3c/webcomponents/commit/ed93413f9a37a6bfbd774cb2e09e14a76c87d56c
BUG=531990

Review URL: https://codereview.chromium.org/1637813002

Cr-Commit-Position: refs/heads/master@{#372596}
8 files changed