webperf: count number of CSS rules, log to Ganglia

* Count the total number of CSS rules that are declared by all stylesheets on
  the current page and log it to Ganglia.
* Count requests / bytes for all resources that are not JS, CSS or image
  requests as 'other'; log to Ganglia.
* Simplify the asset-check.js's output JSON object format, so that it consists
  of resource type keys that map to nested objects with metric keys and integer
  values. This makes the object easier to decode iteratively from Python.

Change-Id: I14c9801d46003c4cdce42e0ca27c51b3640e0cbc