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This change finishes plumbing of device connection events from the devices
to the browser, and fixes handling in the WebXR implementation.

Bug: 789620,778041
Change-Id: Iba9999c0b021261df23add4d1bb2826bd0764ed4
Reviewed-on: https://chromium-review.googlesource.com/c/1364332
Commit-Queue: Bill Orr <billorr@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616118}