H5中获取摄像头或者音频的方法，由于版本的不同，低版本的浏览器navigator.mediaDevices是不可获得的，所以采用这种写法。

var commomUserMedia = function(constraints) {　　var getUserMedia = (　　　　navigator.getUserMedia ||　　　　navigator.webkitGetUserMedia ||　　　　navigator.mozGetUserMedia　　);　　if(!getUserMedia) {    　　return Promise.reject(new Error('getUserMedia is not implemented in this browser'));　　}　　return new Promise(function(resolve, reject) {   　　getUserMedia.call(navigator, constraints, resolve, reject);　　});    }if(navigator.mediaDevices === undefined) {    navigator.mediaDevices = {};}if(navigator.mediaDevices.getUserMedia === undefined) {    navigator.mediaDevices.getUserMedia = commomUserMedia;}var constraints = { audio: true, video: { width: 1280, height: 720 } };navigator.mediaDevices.getUserMedia(constraints).then(function(stream) {    　　var video = document.querySelector('video');    　　video.src = window.URL.createObjectURL(stream);    　　video.onloadedmetadata = function(e) {    　　video.play();　　};}).catch(function(err) {　　console.log(err.name + ": " + err.message);});