Fix variable shadowing warning in safe_sprintf in VS2015

Fix variable shadowing warning in safe_sprintf in VS2015

d:\src\cr3\src\base\strings\safe_sprintf.cc(513): error C2220: warning treated as error - no 'object' file generated
d:\src\cr3\src\base\strings\safe_sprintf.cc(513): warning C4456: declaration of 'ch' hides previous local declaration
d:\src\cr3\src\base\strings\safe_sprintf.cc(450): note: see declaration of 'ch'

R=thestig@chromium.org
BUG=440500
Committed: https://crrev.com/3c598102874c539a4b5e7f9bc51f624f245827dc
Cr-Commit-Position: refs/heads/master@{#313343}

[Lei Zhang, 2015-01-27 19:36:15]

lgtm

[scottmg, 2015-01-27 20:14:35]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2015-01-27 20:16:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/868023005/1

[commit-bot: I haz the power, 2015-01-27 20:22:00]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-27 20:23:14]

Patchset 1 (id:??) landed as
https://crrev.com/3c598102874c539a4b5e7f9bc51f624f245827dc
Cr-Commit-Position: refs/heads/master@{#313343}