This fixes how arguments of a call to {fround} are being parsed. It now
accepts a single "AssignmentExpression" only instead of an "Expression"
which could potentially be a whole comma-separated list of expressions.

R=clemensh@chromium.org
TEST=mjsunit/regress/regress-crbug-1006592
BUG=chromium:1006592

Change-Id: Ifaf0c2b048e4ec18429cc6039c0e7dcdecc1d0bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1815255Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63926}