Fix crash when focus-navigating to a shadow insertion point in the oldest shadow tree.

Shadow insertion points in the oldest shadow tree should not be treated as a focus scope owner.

The relevant spec is: http://w3c.github.io/webcomponents/spec/shadow/#focus-navigation

BUG=336289

Review URL: https://codereview.chromium.org/143643004

git-svn-id: svn://svn.chromium.org/blink/trunk@165476 bbb929c8-8fbe-4397-9dbb-9b2b20218538