Add event object to back end and implement creation and info query.

Bug: angleproject:6001
Change-Id: Id4951b726b66afa289fafe1a716a7cfdb6f5826d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2925477
Commit-Queue: John Plate <jplate@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>