Split content script injections into multiple tasks

chrome/test/data/extensions/api_test/messaging/connect/manifest.json

 {
   "name": "connect_external",
   "version": "1.0",
   "manifest_version": 2,
   "description": "Tests connect from a content script.",
   "permissions": ["tabs", "webNavigation"],
   "background": {
     "persistent": false,
     "scripts": ["test.js"]
   },
   "content_scripts": [
     {
       "matches": ["http://*/*"],
+      "run_at": "document_end",
       "js": ["page.js"]
     },
     {
       "all_frames": true,
       "matches": [
         "http://*/*?testSendMessageFromFrame*",
         "http://*/*?testConnectChildFrameAndNavigate*"
       ],
       "js": ["frame.js"]
     }
   ]
 }