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This will help as we add more events for additional functionality.
Particular flags need to be set for all CustomEvent's so they bubble
up through the Polymer components.

Change-Id: I11e909b7a6afc6bc9433392e1fbb160156cdf10e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636077
Commit-Queue: Kenton Lam <kentonlam@google.com>
Reviewed-by: Keith Lee <keithlee@chromium.org>
Reviewed-by: John Palmer <jopalmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844672}