Remove SiblingRuleHelper

Since the bits are actually stored in NodeRareData anyway, we should
just use those same bits for ShadowRoot and get rid of all this code
duplication.

Review URL: https://codereview.chromium.org/208933003

git-svn-id: svn://svn.chromium.org/blink/trunk@169787 bbb929c8-8fbe-4397-9dbb-9b2b20218538