Rename Event.scoped to Event.composed with default being false

See https://dom.spec.whatwg.org/#dom-event-composed for the spec.
See https://github.com/whatwg/html/issues/1160 for the context.

For histograms,
TBR=isherman@chromium.org

BUG=531990

Review-Url: https://codereview.chromium.org/2012423004
Cr-Commit-Position: refs/heads/master@{#396819}
22 files changed