This prevents unnecessary switching back and forth between internal and
public API boundaries. It is also a step towards making all WebAssembly
internals completely independent of "scheduled exception" values.

R=ahaas@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I46e8cb4fad3d255d9bd20b9c343901a03a25426c
Reviewed-on: https://chromium-review.googlesource.com/895742Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51002}