Revert of Make initEvent's first argument non-optional (patchset #4 id:60001 of https://codereview.chromium.org/2579993002/ )

Reason for revert:
It looks this broke the external/wpt/dom/interfaces.html test:

https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.12/builds/448
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.12/builds/438
...

Original issue's description:
> Make initEvent's first argument non-optional
>
> https://github.com/whatwg/dom/issues/387
>
> BUG=673698
>
> Review-Url: https://codereview.chromium.org/2579993002
> Cr-Commit-Position: refs/heads/master@{#458108}
> Committed: https://chromium.googlesource.com/chromium/src/+/2948f11e04fba5f0dc7244229c542152c97c1624

TBR=foolip@chromium.org,lunalu@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=673698

Review-Url: https://codereview.chromium.org/2760173002
Cr-Commit-Position: refs/heads/master@{#458227}
6 files changed