<dialog>: Use the composed shadow tree when testing inertness.

This also fixes a bug where the ::backdrop is considered
inert when <dialog> has no children, due to the way
Node::containsIncludingShadowDOM works.

BUG=269842

Review URL: https://codereview.chromium.org/145503006

git-svn-id: svn://svn.chromium.org/blink/trunk@166375 bbb929c8-8fbe-4397-9dbb-9b2b20218538