This is a reland of e084eea6.
Undefined behavious was fixed in https://crrev.com/c/1051235.

Original change's description:
> Fix SourcePositionInfo for wasm
>
> In wasm we often don't have a SharedFunctionInfo associated with a
> compilation job, so we can't get a Script. Just print "unknown" in
> these cases (instead of crashing).
>
> R=titzer@chromium.org
> CC=​herhut@chromium.org
>
> Bug: chromium:840757, v8:7738
> Change-Id: I850c6adfd9e07c9a0f6dd018f1a9314feb89d887
> Reviewed-on: https://chromium-review.googlesource.com/1049632
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53080}

TBR=titzer@chromium.org

Bug: chromium:840757, v8:7738
Change-Id: If04040a33766955cfed78e7c27226dd04c3f9b9f
Reviewed-on: https://chromium-review.googlesource.com/1051266Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53111}