In debug mode (SLOW_DCHECK), Handle<T>::cast accesses the object to
check its type.  Obviously we can no longer do that now that we run
on a background thread.

NOTE: I think there are other parts of TF that suffer from the same
problem. I will look into fixing those as well.

Bug: v8:6048, v8:6590
Change-Id: I9abfdf30f1899cdb0c8b9078b0cf71463d608251
Reviewed-on: https://chromium-review.googlesource.com/570054Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46641}