修复BroadCastChannel内存泄露问题 (#3288)

在同页面中不停的切换，导致内存无法释放，代码片段如下：
data(){
    return {
        BroadcastNotice:new BroadcastChannel('hundsun-notice111111')
    }
},
created() {
    this.BroadcastNotice.onmessage = event => this.$emit("onNoticeMsg", event) //泄露
    // this.HundsunNotice.onmessage = event => {}  //正常

},

runtime/services/broadcast-channel/index.js

@@ -69,6 +69,7 @@ BroadcastChannel.prototype.close = function () {
   }
 
   this._closed = true
+  this.onmessage = null
 
   // remove itself from channels.
   if (channels[this.name]) {