fix(background.js) take descriptor from correct place

When opening dev tools for the first time the scopeTree did not show
descriptors.
This was caused by the code looking in the wrong place for the
descriptor.

Author: gary-b

background.js

@@ -65,7 +65,7 @@ function bufferData(tabId, message) {
       scope.models[message.data.path] = (typeof message.data.value === 'undefined') ?
                                             undefined : message.data.value;
     } else if (message.event === 'scope:link') {
-      scope.descriptor = message.descriptor;
+      scope.descriptor = message.data.descriptor;
     }
   }