Make initEvent's first argument non-optional

https://github.com/whatwg/dom/issues/387

BUG=673698

Review-Url: https://codereview.chromium.org/2579993002
Cr-Original-Commit-Position: refs/heads/master@{#458108}
Committed: https://chromium.googlesource.com/chromium/src/+/2948f11e04fba5f0dc7244229c542152c97c1624
Review-Url: https://codereview.chromium.org/2579993002
Cr-Commit-Position: refs/heads/master@{#458443}
6 files changed