Deprecate invoking the default action for untrusted events.

Add deprecation for dispatching untrusted events.

Intent to deprecate: https://groups.google.com/a/chromium.org/d/msg/blink-dev/OzHI2c2vkEo/9Bz-j4PhBAAJ
Chrome status: https://www.chromestatus.com/features/5718803933560832

BUG=520519

Review-Url: https://codereview.chromium.org/1989463002
Cr-Commit-Position: refs/heads/master@{#394479}
6 files changed