Making :host in :host's parameter match shadow host.

Adding one more behaviorAtBoundary, TreatShadowHostAsNormalScope, to change isHostInItsShadowTree's behavior.

BUG=313935
TEST=fast/dom/shadow/pseudo-host-parameter-matches-shadow-host.html

Review URL: https://codereview.chromium.org/62333007

git-svn-id: svn://svn.chromium.org/blink/trunk@161703 bbb929c8-8fbe-4397-9dbb-9b2b20218538