There's no need to return anything.

Also add a DCHECK to make sure that we never mark a variable proxy as assigned
that is already resolved (to avoid potential inconsistency with the variable's
maybe_assigned status).

R=littledan@chromium.org
CC=mstarzinger@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2504613002
Cr-Commit-Position: refs/heads/master@{#41072}