This change has been useful when inspecting on_reject callbacks in
Isolate::InternalPromiseHasUserDefinedRejectHandler, it's probably
going to be helpful for other things too.

BUG=
R=adamk@chromium.org, littledan@chromium.org

Change-Id: I7c5bac87a3b8bd3ce19d71cf73397a93109976ba
Reviewed-on: https://chromium-review.googlesource.com/907641Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#51164}