Port f71e2626

Original commit message:
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}

R=danno@chromium.org, svenpanne@chromium.org

BUG=

Review URL: https://codereview.chromium.org/999093002

Cr-Commit-Position: refs/heads/master@{#27144}