原理：設立時間戳，全域性設定監聽，把滅屏的時間計算出來，再次亮屏時加上該段時間

var betweenTime = 0;
var b=0;
var num = 0;
document.addEventListener('webkitvisibilitychange',function()
{
    if(document.webkitVisibilityState=='hidden')
    {
        b=Date.now();
}else
{
        var betweenMs = Date.now() - b;
var  betweens = Math.floor(betweenMs / 1000
 
);
betweenTime = Math.floor(betweens / 60);
console.log('間隔:' + betweenTime + '分鐘');
num=betweenTime;
}
});