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

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State at 'canplaythrough' event:

    title: AXTitle: 
    role: AXRole: AXVideo


        title: AXTitle: Video Playback
        role: AXRole: AXToolbar


            title: AXTitle: Play
            role: AXRole: AXButton


            title: AXTitle: Duration
            role: AXRole: AXSlider


            title: AXTitle: Remaining 6 Seconds
            role: AXRole: AXTimer


            title: AXTitle: Display Full Screen
            role: AXRole: AXButton


            title: AXTitle: Mute
            role: AXRole: AXCheckbox


            title: AXTitle: Volume
            role: AXRole: AXSlider