Clean up SelectorChecker::checkOne()

Split SelectorChecker::checkOne() into
checkPseudoClass(), checkPseudoElement() and
checkPseudoHost() to be more readable.

BUG=none
TEST=do not break layout tests

Review URL: https://codereview.chromium.org/511023003

git-svn-id: svn://svn.chromium.org/blink/trunk@183305 bbb929c8-8fbe-4397-9dbb-9b2b20218538