The ScriptDetails passed into a compilation may include a source
mappping URL. We should make sure that this does not overwrite an
existing source mapping URL on a script object, which will have been
inferred from source (and will take priority).

Bug: chromium:1011762
Change-Id: I5591005f86087b635764be980e00db76ed2a826d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2126912
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67401}