HTMLLIElement and RenderListItem should consider distribution.

BUG=339754
TEST=fast/dom/shadow/shadow-and-list-elements.html,w3c/web-platform-tests/shadow-dom/shadow-trees/lower-boundary-encapsulation/distribution-001.html,w3c/web-platform-tests/shadow-trees/lower-boundary-encapsulation/distribution-002.html

Review URL: https://codereview.chromium.org/169273003

git-svn-id: svn://svn.chromium.org/blink/trunk@169234 bbb929c8-8fbe-4397-9dbb-9b2b20218538