MIPS64: Fix string stubs.

MIPS64: Fix string stubs.

An innocent-looking change in 'c63e50ed Reland Update V8 DEPS.' exposed
latent bugs in SubStringStub and StringCharFromCodeGenerator.

TEST=mjsunit/string-replace, msjunit/string-split, others....
BUG=

Committed: https://crrev.com/88214f1bf8429311d26d8f85f2a1e4759cb3fe7e
Cr-Commit-Position: refs/heads/master@{#29731}

[paul.l..., 2015-07-17 03:54:28]

paul.lind@imgtec.com changed reviewers:
+ balazs.kilvady@imgtec.com, dusan.milosavljevic@imgtec.com

[paul.l..., 2015-07-17 03:54:29]

PTAL.

[balazs.kilvady, 2015-07-17 09:15:22]

LGTM.

[dusmil.imgtec, 2015-07-17 09:43:11]

LGTM.

[paul.l..., 2015-07-17 13:24:02]

The CQ bit was checked by paul.lind@imgtec.com

[commit-bot: I haz the power, 2015-07-17 13:24:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1233903003/1

[commit-bot: I haz the power, 2015-07-17 13:56:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-17 13:57:11]

Patchset 1 (id:??) landed as
https://crrev.com/88214f1bf8429311d26d8f85f2a1e4759cb3fe7e
Cr-Commit-Position: refs/heads/master@{#29731}