Simplify SelectRuleFeatureSet class.

Subclass SelectRuleFeatureSet from RuleFeatureSet instead of having it as a
member. Also make use of the parsePseudo method to simplify the Internals
helper.

Review URL: https://codereview.chromium.org/598093005

git-svn-id: svn://svn.chromium.org/blink/trunk@182746 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed