Implements getDestinationInsertionPoints() API behind the Shadow DOM flag.

The spec is here:
https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#api-partial-element-get-destination-insertion-points

BUG=234031
R=dglazkov@chromium.org, esprehn@chromium.org, yutak@chromium.org

Review URL: https://codereview.chromium.org/22335003

git-svn-id: svn://svn.chromium.org/blink/trunk@155735 bbb929c8-8fbe-4397-9dbb-9b2b20218538