Enable do not allow default action for untrusted events.

Approved intent to ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/WEMnlVIbe70/i2MJcEtNBgAJ

BUG=565760,423975,520519

Review-Url: https://codereview.chromium.org/2070053004
Cr-Commit-Position: refs/heads/master@{#401463}
15 files changed