Fix assertion fail in htmlediting.

r162687 causes this assertion, because the revision broke the assumption that
the node owning a renderer is editable.

BUG=343037
TEST=editing/selection/table-element-crash.html

Review URL: https://codereview.chromium.org/130613006

git-svn-id: svn://svn.chromium.org/blink/trunk@167191 bbb929c8-8fbe-4397-9dbb-9b2b20218538