SelectorChecker should update scope and behaviorAtBoundary for /content/ in SharingRules mode.

When ShardStyleFinder checks whether a given element can reuse other element's style
or not, it uses SelectorChecker with SharingRules mode.
However, RuleFeatureSet used in SharingRules mode doesn't have any information about styles' scope, i.e. SelectorCheckingContext's scope.
So Blink should treat as if scope and behavior are set correctly.

BUG=353428
TEST=fast/dom/shadow/style-sharing-with-content-and-host.html

Review URL: https://codereview.chromium.org/203303002

git-svn-id: svn://svn.chromium.org/blink/trunk@170176 bbb929c8-8fbe-4397-9dbb-9b2b20218538