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

Committed: https://crrev.com/f71e26268338d2840133264329bf35a63011bb5a
Cr-Commit-Position: refs/heads/master@{#27127}

[Benedikt Meurer, 2015-03-11 09:42:43]

bmeurer@chromium.org changed reviewers:
+ bmeurer@chromium.org

[Benedikt Meurer, 2015-03-11 09:42:44]

Nice. LGTM

[Michael Starzinger, 2015-03-11 09:52:52]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-03-11 09:53:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/995013005/20001

[commit-bot: I haz the power, 2015-03-11 10:02:47]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-11 10:02:56]

Patchset 2 (id:??) landed as
https://crrev.com/f71e26268338d2840133264329bf35a63011bb5a
Cr-Commit-Position: refs/heads/master@{#27127}