Refactor SelectorChecker::BehaviorAtBoundary

As of Blink r176304, SelectorCheckingContext::scope
for styles in shadow tree changed from its shadow host to
shadow root.

Thus the subtlety of scoping difference between TreeScope
and SelectorCheckingContext::scope is gone, and code for
handling the difference can be cleaned up.

This made possible by dropping support for <style scoped>,
which introduced the complexity at r175993.

With this change no functional difference is expected,
thus all layout tests should pass.

- Convert SelectorChecker::parentElement(context) to static helper function.
- Convert SelectorChecker::scopeContainsLastMatchedElement() to static
  helper function.
- s/{b,B}ehaviorAtBounary/{c,C}ontextFlags/g
- Removed the 2nd parameter in rulesApplicableInCurrentTreeScope()
- Removed the 2nd argument for isHostInItsShadowTree()
- Removed StaysWithinTreeScope
- Removed DoesNotCrossBoundary
- Removed ScopeIsShadowRoot.
- Removed BoundaryBehaviorMask
- Removed ScopeIsShadowHostInPseudoHostParameter
- Introduced DefaultBehavior (to be removed, though)

BUG=none
TEST=pass all layout tests.

Review URL: https://codereview.chromium.org/334263020

git-svn-id: svn://svn.chromium.org/blink/trunk@177183 bbb929c8-8fbe-4397-9dbb-9b2b20218538