port f71e2626 (r27127)

original commit message:

  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.

BUG=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/1028993003

Cr-Commit-Position: refs/heads/master@{#27358}