MIPS: Simplify pending message script handling.

MIPS: Simplify pending message script handling.

Port f71e26268338d2840133264329bf35a63011bb5a

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.

BUG=

Committed: https://crrev.com/2bd426cdd5eabd37a35ea471bd298072a1257410
Cr-Commit-Position: refs/heads/master@{#27140}

[balazs.kilvady, 2015-03-11 16:43:14]

balazs.kilvady@imgtec.com changed reviewers:
+ akos.palfi@imgtec.com, danno@chromium.org, dusan.milosavljevic@imgtec.com,
gergely.kis@imgtec.com, mstarzinger@chromium.org, paul.lind@imgtec.com

[paul.l..., 2015-03-11 16:52:02]

LGTM.

[balazs.kilvady, 2015-03-11 16:55:46]

The CQ bit was checked by balazs.kilvady@imgtec.com

[commit-bot: I haz the power, 2015-03-11 16:56:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/999763003/1

[commit-bot: I haz the power, 2015-03-11 17:25:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-11 17:25:23]

Patchset 1 (id:??) landed as
https://crrev.com/2bd426cdd5eabd37a35ea471bd298072a1257410
Cr-Commit-Position: refs/heads/master@{#27140}