title: Chrome extension getUrl not working in injected file
body: <p>I'm developing a Chrome extension, is there any way to get the <code>chrome.extension.getURL('file path')</code> method from injected file? I'm unable to access above method from injected file.</p>
  
  <p>manifest.json</p>
  
  <pre><code>{
  "name": "Name",
  "version": "0.1",
  "description": "Name chrome extension",
  "background": {
  "persistent": false,
  "scripts": ["js/background.js"]
  },
  "permissions": [
  "tabs", 
  "https://*/*"
  ],
  "content_scripts": [
  {
    "matches": ["https://mail.google.com/*"],
    "js": ["js/content.js"],
    "run_at": "document_end"
  }
  ],
  "web_accessible_resources": [
  "js/injected.js",
  "html/popup.html"
  ],
  "manifest_version": 2
  }
  </code></pre>
  
  <p>injected.js</p>
  
  <pre><code>console.log("Ok injected file worked");
  console.log("Url: ", chrome.extension.getURL('html/popup.html'));
  </code></pre>
  
  <p>contentScript.js</p>
  
  <pre><code>var s = document.createElement('script');
  s.src = chrome.extension.getURL('js/injected.js');
  (document.head || document.documentElement).appendChild(s);
  </code></pre>