For wasm modules with non-absolute sourceMappingURL, the source needs
to be empty so that devtools can look for the source map at the origin
of the module.

R=clemensh@chromium.org,adamk@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I74c40addc1a7cb1be0442e9f2b272590c0b81f60
Reviewed-on: https://chromium-review.googlesource.com/1250402
Commit-Queue: Aseem Garg <aseemgarg@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56326}