Removes script() from CompilationInfo since it might not be created when
compiling from a background thread.

BUG=v8:5203

Change-Id: Ic36fd04cf4792336707b2d3715d47c59b6a97faf
Reviewed-on: https://chromium-review.googlesource.com/690299Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48220}