Use a HashCountedSet to count identifiers in InspectorConsoleAgent

Use a HashCountedSet to count identifiers in InspectorConsoleAgent instead of
handling the counting manually and using a HashMap data structure. This
abstracts away some of the complexity and makes the code a bit simpler.

Review URL: https://codereview.chromium.org/69343003

git-svn-id: svn://svn.chromium.org/blink/trunk@161811 bbb929c8-8fbe-4397-9dbb-9b2b20218538