GitLab

Add type for all CustomEvent parameters and remove unnecessary type
casts.

Bug: 924733
Change-Id: I3b521ab2ca575aa3caf59921d06a2af75361c2fd
Reviewed-on: https://chromium-review.googlesource.com/c/1456695
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629892}