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This is the object passed to onconnected and ondisconnected event
handlers connected to navigator.usb.

BUG=492204

Review URL: https://codereview.chromium.org/1307483002

git-svn-id: svn://svn.chromium.org/blink/trunk@200935 bbb929c8-8fbe-4397-9dbb-9b2b20218538