Abstract away some of the complexity in CSSSelectorWatch by using HashCountedSet

Abstract away some of the complexity in CSSSelectorWatch by using HashCountedSet data
structure instead of handling the counting manually and use a HashMap.
WTF's HashCountedSet exists exactly for this kind of use cases and it makes the code
a bit simpler.

R=eseidel

Review URL: https://codereview.chromium.org/69543002

git-svn-id: svn://svn.chromium.org/blink/trunk@161838 bbb929c8-8fbe-4397-9dbb-9b2b20218538