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}