Make <shadow> elements have getDistributedNodes()

- The issue on the spec side is: https://www.w3.org/Bugs/Public/show_bug.cgi?id=23052
- The spec is: https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#api-html-shadow-element-get-distributed-nodes

BUG=311454

Review URL: https://codereview.chromium.org/42993002

git-svn-id: svn://svn.chromium.org/blink/trunk@160689 bbb929c8-8fbe-4397-9dbb-9b2b20218538