Also store the variable directly on ClassLiteral, as the proxy serves
as a useless form of indirection.

Bug: v8:6092
Change-Id: If0182a808cde4e349c1bf5a003a1ecee5bd14b13
Reviewed-on: https://chromium-review.googlesource.com/667800Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48072}