Use CORS for fetching NaCl resources from extensions.

ppapi/native_client/tests/ppapi_browser/extension_mime_handler/manifest.json

 {
   "name": "NaCl_MIMETest",
   "version": "0.1",
   "manifest_version": 2,
   "description": "Tests NaCl MIME type handling capabilities.",
   "permissions": [
     "experimental",
     "nativeclient"
   ],
+  "web_accessible_resources": [ "ppapi_extension_mime_handler.nmf",
+                                "ppapi_extension_mime_handler_x86-32.nexe",
+                                "ppapi_extension_mime_handler_x86-64.nexe",
+                                "ppapi_extension_mime_handler_arm.nexe" ],
   "nacl_modules": [{
     "path": "ppapi_extension_mime_handler.nmf",
     "mime_type": "foo/bar"
   }]
 }