A map’s `constructor_or_backpointer` can be any kind of value,
because `fn.prototype = foo` sets that field to `foo` if the
latter is not a `JSReceiver`; so the `DCHECK` that is being
removed here was invalid.

Refs: https://github.com/nodejs/node/issues/18223
Bug: node:18223
Change-Id: Ia6449c07bb724e515d73b162369ab36ab1d89c6b
Reviewed-on: https://chromium-review.googlesource.com/874472
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50735}