Simplify pending message script handling.

Simplify pending message script handling.

This removes the separate tracking of the pending message script,
because that script is already stored in the message object and
duplicating it in the ThreadLocalTop makes it more brittle.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/995013005

Cr-Commit-Position: refs/heads/master@{#27127}