Clean up registration context state on element death.

Custom Element registration contexts keep the elements they created
which are waiting for a definition around in an "upgrade candidate
map." If the element moves to a different document but subsequently
dies, that state has to be cleaned up if the registration context is
still around.

BUG=255725
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/19667005

git-svn-id: svn://svn.chromium.org/blink/trunk@154870 bbb929c8-8fbe-4397-9dbb-9b2b20218538