We now unconditionally both parse and compile StreamedSource on the background
thread.

BUG=v8:5203

Change-Id: I42d6fe9059bc1745da3a415d270f46cf1c08b306
Reviewed-on: https://chromium-review.googlesource.com/948854Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51758}