Use HashCountedSet for fontStats in InspectorCSSAgent

Use HashCountedSet for fontStats in InspectorCSSAgent instead of handling the
counting manually and using a HashMap data structure. This makes the code a
bit simpler.

Review URL: https://codereview.chromium.org/68243006

git-svn-id: svn://svn.chromium.org/blink/trunk@161837 bbb929c8-8fbe-4397-9dbb-9b2b20218538