带有dom元素的chrome扩展程序更新徽章

Question

所以只想在页面上计算标签并在徽章上显示

content.js

document.body.addEventListener('DOMContentLoaded', e => {
var a=document.getElementsByTagName('a').length;

    chrome.runtime.sendMessage(a)

});

background.js

chrome.runtime.onMessage.addListener(message => {
var b=message
chrome.browserAction.setBadgeText({"text":b.toString()})

});

manifest.json

{
"manifest_version": 2,
"name": "The Chrome Link Counter",
"version": "1.0",
"description": "counter",
"icons": {
    "48": "icons/icon_default.png"
},
"content_scripts": [
    {
        "matches": ["<all_urls>"],
        "all_frames": true,
        "js": ["content/main.js"]
    }
],
"background": {
    "scripts": ["background/main.js"]
},
"options_ui": {
    "page": "options/main.html",
    "chrome_style": true
},
"browser_action": {
    "default_popup": "options/main.html",
    "chrome_style": true,
    "default_icon": {
        "16": "icons/icon_default.png"
    }
},
"web_accessible_resources": ["icons/icon.png", "icons/check.png"],
"permissions": ["storage", "webRequest","tabs"]

}

但是当我尝试这里缺少的东西时什么都没有发生？

编辑：还添加了manifest.json