When streaming compilation for WebAssembly gets aborted, we reject the
promise associated with the compilation. However, in some circumstances,
e.g. when streaming comilation gets aborted because the browser tab gets
refreshed, then we want to omit rejecting the promise. In an older CL
(https://crrev.com/c/876103) we omit rejecting the promise when the
exception value is null. With this CL the exception value is a MaybeLocal
so that we document properly that the value can be null. In addition, I
added documentation to say that in that case we do not reject the promise.

R=adamk@chromium.org

Bug: chromium:803838
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I6a093e61c8ec63f7ae385a7f77ae6178e7b34a06
Reviewed-on: https://chromium-review.googlesource.com/897647Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51211}