Don't try to find a slot in a v0 shadow tree.

ElementShadow::assignedSlotFor asserts that it's only called for V1
ElementShadows and doesn't null check m_slotAssignment. Guard with an
if-check for V1 when assignedSlotFor from Node::assignedSlot like we
already do from Node::assignedSlotForBinding.

R=hayato@chromium.org,kochi@chromium.org
BUG=588209

Review URL: https://codereview.chromium.org/1717053002

Cr-Commit-Position: refs/heads/master@{#376699}
3 files changed