Remove SelectorChecker::ContextFlags

Expand SelectorChecker::ContextFlags into SelectorCheckingContext to
shrink thie size of SelectorCheckingContext from
72bytes to 64bytes on 64bit platforms.

BUG=none
TEST=do not break layout tests

Review URL: https://codereview.chromium.org/631713002

git-svn-id: svn://svn.chromium.org/blink/trunk@183321 bbb929c8-8fbe-4397-9dbb-9b2b20218538