Fix shadowing warning in json_parser on VS2015

Fix shadowing warning in json_parser on VS2015

d:\src\cr3\src\base\json\json_parser.cc(431): error C2220: warning treated as error - no 'object' file generated
d:\src\cr3\src\base\json\json_parser.cc(431): warning C4456: declaration of 'next_char' hides previous local declaration
d:\src\cr3\src\base\json\json_parser.cc(427): note: see declaration of 'next_char'

I don't think there's any need for a new variable inside the loop.

R=thestig@chromium.org
BUG=440500
Committed: https://crrev.com/e5937efe673d362d4c933b2194fa808e32167de1
Cr-Commit-Position: refs/heads/master@{#313374}

[Lei Zhang, 2015-01-27 19:30:27]

lgtm

[scottmg, 2015-01-27 19:31:13]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2015-01-27 19:32:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/881003003/1

[commit-bot: I haz the power, 2015-01-27 20:47:05]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-01-27 20:47:06]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[scottmg, 2015-01-27 20:50:33]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2015-01-27 20:51:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/881003003/1

[commit-bot: I haz the power, 2015-01-27 20:52:25]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-01-27 20:52:26]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[scottmg, 2015-01-27 22:09:23]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2015-01-27 22:10:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/881003003/1

[commit-bot: I haz the power, 2015-01-27 22:12:51]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-27 22:14:21]

Patchset 1 (id:??) landed as
https://crrev.com/e5937efe673d362d4c933b2194fa808e32167de1
Cr-Commit-Position: refs/heads/master@{#313374}