The DCHECK is firing because the fuzzer doesn't set any
HostImportModuleDynamically callback.  Previously
RunHostImportModuleDynamicallyCallback would not assert for this and
would just return a rejected promise.  After
https://chromium-review.googlesource.com/c/v8/v8/+/2620578, this results
in a failed DCHECK.

This change restores the old behavior by loosening the DCHECK such that
it only fails if both the deprecated and the new callback are set.

Bug: chromium:1172121
Change-Id: Ifda28eb28572a40d3752928997edf25d607b61c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2659505Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#72462}