Move CSSSelectorWatch related code out of recalcStyle methods

Instead of having checks all over the code we can refactor them into methods.

Review URL: https://codereview.chromium.org/68183007

git-svn-id: svn://svn.chromium.org/blink/trunk@161789 bbb929c8-8fbe-4397-9dbb-9b2b20218538