Don't expose HTMLSlotElement if Shadow DOM V1 is not enabled.

Invoking toString on the element serialized to [object HTMLSlotElement]
which is not correct until we ship.

R=hayato@chromium.org,kochi@chromium.org
BUG=531990

Review URL: https://codereview.chromium.org/1717823002

Cr-Commit-Position: refs/heads/master@{#376701}
1 file changed