Android Pie unable to use front camera

[74jian]

recommended add camera settings,because mobile devices use Face (camera) need front camera.and the current code does not support Android Pie(9) using the front camera.because Android P use front camera requires getUserMedia to force use facingMode: { exact: "user" } or use deviceId to select,this is my solution for use deviceId in Android Pie(9)

tracking.initUserMedia_ = function(element, opt_options) {
    window.navigator.mediaDevices.enumerateDevices().then(function(devices) {
		let cfg={
			video:{'facingMode':'user'}, 
			audio: (opt_options && opt_options.audio) ? true : false,
		};
		devices = devices.filter(function(device) { return device.kind === 'videoinput'});
		if (navigator.userAgent.toLowerCase().indexOf("android") > 0) {
			for (let i = 0; i < devices.length; i++) {
				let device = devices[i];
				if (device.label) {
					if (device.label.split(',')[1]==' facing front'){
						cfg={
							video:{ 
								deviceId: {'exact':device.deviceId},
							},
							audio: (opt_options && opt_options.audio) ? true : false ,
						};
						break;
					}
				}
			}
		}
		window.navigator.mediaDevices.getUserMedia(cfg).then(function(stream) {
			element.srcObject = stream;
		}).catch(function(err) {
			throw Error('Cannot capture user camera.');
		});
	});
  };