Dump <video> element controller accessibility object tree at 'canplaythrough' event.

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State at 'canplaythrough' event:

    description: AXDescription: video element controller
    role: AXRole: tool bar


        description: AXDescription: play
        role: AXRole: push button


        description: AXDescription: 
        role: AXRole: slider


        description: AXDescription: fast reverse
        role: AXRole: push button


        description: AXDescription: fast forward
        role: AXRole: push button


        description: AXDescription: enter fullscreen
        role: AXRole: push button


        description: AXDescription: mute
        role: AXRole: push button