Allow content script insertion on about:-URLs.

chrome/test/data/extensions/api_test/content_scripts/about_blank_iframes/manifest.json

 {
   "name": "about:blank iframes",
   "version": "1.0",
   "manifest_version": 2,
   "description": "Tests that content scripts are not injected into about:blank iframes, even after document.write().",
   "background": {
     "page": "test.html"
   },
   "permissions": ["tabs"],
   "content_scripts": [
     {
       "matches": ["http://*/*"],
       "js": ["content_script.js"],
       "all_frames": true
+    },
+    {
+      "run_at": "document_start",
+      "matches": ["http://does-not-match/"],
+      "match_about_blank": true,
+      "js": ["content_script2.js"],
+      "all_frames": true
     }
   ]
 }