Remove SelectorCheckerFastPath

There's no evidence that this fast path actually matters, the only difference 
seems to be the number of if and switch statements. If it does we should look 
into how we can make the regular SelectorChecker faster instead of having two modes.

Review URL: https://codereview.chromium.org/208423010

git-svn-id: svn://svn.chromium.org/blink/trunk@169993 bbb929c8-8fbe-4397-9dbb-9b2b20218538