Split content script injections into multiple tasks

chrome/test/data/extensions/api_test/clipboard/extension_no_permission/manifest.json

 {
   "name": "chrome.clipboard.extension_no_permission",
   "version": "0.1",
   "manifest_version": 2,
   "description": "end-to-end browser test for clipboard permissions",
   "background": {
     "page": "test.html"
   },
   "permissions": ["http://localhost/*"],
   "content_scripts": [
     {
       "matches": ["http://*/*test_file_with_body.html"],
+      "run_at": "document_end",
       "js": ["content_script.js"]
     }
   ]
 }