Stop passing around 'scopeContainsLastMatchedElement' flag.

Hold the |scopeContainsLastMatchedElement| flag in ElementRuleCollector class
so that during CSS rule collection the flag doesn't have to be passed around.

This CL depends on
https://codereview.chromium.org/631713002/

BUG=none
TEST=do not break laoyout tests

Review URL: https://codereview.chromium.org/636463002

git-svn-id: svn://svn.chromium.org/blink/trunk@183389 bbb929c8-8fbe-4397-9dbb-9b2b20218538