GitLab

Before this CL we used V8PrivateProperty to avoid
cyclic references between V8 and Blink. Now we have
trace wrapper and DOM object can have a reference to
V8 objects. Use trace wrapper for CustomEvent

BUG=501866,700680

Change-Id: I9089d9cf0828bb979467cc5bd859e460847b6c28
Reviewed-on: https://chromium-review.googlesource.com/484162
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#467934}