Fix Histogram shadowing warning on vs2015

Fix Histogram shadowing warning on vs2015

d:\src\cr3\src\base\metrics\histogram.cc(498): warning C4457: declaration of 'count' hides function parameter
d:\src\cr3\src\base\metrics\histogram.cc(490): note: see declaration of 'count'

R=isherman@chromium.org
BUG=440500
Committed: https://crrev.com/dcc933d085af63675b27aa499717b97deb5489a2
Cr-Commit-Position: refs/heads/master@{#313361}

[scottmg, 2015-01-27 18:46:35]

scottmg@chromium.org changed reviewers:
+ isherman@chromium.org

[Ilya Sherman, 2015-01-27 21:33:26]

LGTM, thanks.

[Ilya Sherman, 2015-01-27 21:33:43]

The CQ bit was checked by isherman@chromium.org

[commit-bot: I haz the power, 2015-01-27 21:34:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/878223002/1

[commit-bot: I haz the power, 2015-01-27 21:38:27]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-27 21:39:29]

Patchset 1 (id:??) landed as
https://crrev.com/dcc933d085af63675b27aa499717b97deb5489a2
Cr-Commit-Position: refs/heads/master@{#313361}