There maybe one more ScriptContext in script_context_table if mksnapshot
executed with embeded script.

R=yangguo@chromium.org

Bug: v8:7304
Change-Id: I6f431f76d4beae61a519c3acc3b856d1f7479740
Reviewed-on: https://chromium-review.googlesource.com/866630Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50637}