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Spec is described here: http://w3c.github.io/webcomponents/spec/shadow/#dfn-scoped-flag
Only with UA's creation of certain events(abort, error, etc.), this scoped flag is set to true.
Note that any event created by users has scoped flag set to false by default.
R=hayato@chromium.org
BUG=531990, 575042

Review URL: https://codereview.chromium.org/1586563005

Cr-Commit-Position: refs/heads/master@{#371747}