cros: Add a header for gaia auth requests.

chrome/browser/resources/gaia_auth/manifest_test.json

 {
   // chrome-extension://mfffpogegjflfpflabcdkioaeobkgjik/
   "key": "MIGdMA0GCSqGSIb3DQEBAQUAA4GLADCBhwKBgQC4L17nAfeTd6Xhtx96WhQ6DSr8KdHeQmfzgCkieKLCgUkWdwB9G1DCuh0EPMDn1MdtSwUAT7xE36APEzi0X/UpKjOVyX8tCC3aQcLoRAE0aJAvCcGwK7qIaQaczHmHKvPC2lrRdzSoMMTC5esvHX+ZqIBMi123FOL0dGW6OPKzIwIBIw==",
   "name": "GaiaTestAuthExtension",
   "version": "0.0.1",
   "manifest_version": 2,
   "content_security_policy": "default-src 'self'; script-src 'self'; frame-src 'self' https://insecure.com https://www.google.com/accounts/ https://accounts.google.com https://accounts.youtube.com https://gaiastaging.corp.google.com",
   "description": "GAIA Test Component Extension",
   "content_scripts": [
     {
       "matches": [
           "https://www.google.com/accounts/*",
           "https://accounts.google.com/*",
           "https://gaiastaging.corp.google.com/*",
           "https://insecure.com/*"
       ],
       "js": ["test/content.js"],
       "all_frames": true
     }
   ],
+  "background": {
+    "scripts": ["background.js"]
+  },
   "web_accessible_resources": [
     "main.css",
     "main.html",
     "main.js",
     "offline.css",
     "offline.html",
     "offline.js",
     "success.html",
     "success.js",
     "util.js",
     "test/content.js"
   ],
   "permissions": [
-      "https://insecure.com/*",
-      "https://www.google.com/accounts/*",
-      "https://accounts.google.com/*",
-      "https://gaiastaging.corp.google.com/*",
-      "chrome://oobe/"
+    "background",
+    "webRequest",
+    "webRequestBlocking",
+    "<all_urls>"
   ]
 }