Add default constructor to the webrtc_overrides Event class.

Needed to add constructor and make use of it in webrtc, see cl
https://webrtc-review.googlesource.com/c/src/+/109500.

Bug: webrtc:9962
Change-Id: I1437a25db26ed5fa5440f5aa22daa2bdd786b73d
Reviewed-on: https://chromium-review.googlesource.com/c/1318892
Reviewed-by: Henrik Grunell <grunell@chromium.org>
Commit-Queue: Niels Möller <nisse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605984}