Make ThreadData::random_number_ an unsigned integer so that overflow has defined behavior.

Make ThreadData::random_number_ an unsigned integer so that overflow has defined behavior.
The problem has been spotted by UBSan in base_unittests:

[ RUN      ] FileProxyTest.SetLength_Expand
../../base/tracked_objects.cc:457:18: runtime error: signed integer overflow: 1517486279 + 2147483647 cannot be represented in type 'int'
[       OK ] FileProxyTest.SetLength_Expand (2 ms)

R=jar@chromium.org
TBR=darin@chromium.org
BUG=none

Committed: https://crrev.com/f7812575621806b0994ff4f96db61de3c0962fd1
Cr-Commit-Position: refs/heads/master@{#303893}

[Alexander Potapenko, 2014-11-12 09:17:33]

Please take a look.

[jar (doing other things), 2014-11-12 17:06:49]

The CQ bit was checked by jar@chromium.org

[jar (doing other things), 2014-11-12 17:06:49]

lgtm

[commit-bot: I haz the power, 2014-11-12 17:07:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/715223002/1

[commit-bot: I haz the power, 2014-11-12 17:12:02]

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

[commit-bot: I haz the power, 2014-11-12 17:12:03]

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

[Alexander Potapenko, 2014-11-12 21:00:49]

glider@chromium.org changed reviewers:
+ darin@chromium.org

[Alexander Potapenko, 2014-11-12 21:01:36]

On 2014/11/12 17:12:03, I haz the power (commit-bot) wrote:
> Try jobs failed on following builders:
>   chromium_presubmit on tryserver.chromium.linux
>
(http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

TBRring Darin

[Alexander Potapenko, 2014-11-12 21:01:46]

The CQ bit was checked by glider@chromium.org

[commit-bot: I haz the power, 2014-11-12 21:02:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/715223002/1

[commit-bot: I haz the power, 2014-11-12 21:05:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-12 21:06:25]

Patchset 1 (id:??) landed as
https://crrev.com/f7812575621806b0994ff4f96db61de3c0962fd1
Cr-Commit-Position: refs/heads/master@{#303893}