A computed property can never be a private field. That's a SyntaxError.

Change the check to a DCHECK.

Bug: v8:5368
Change-Id: I6701b60f3193639f3ccffacda25074d32d5de5b8
Reviewed-on: https://chromium-review.googlesource.com/c/1385267Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58451}