Make GUID generation on POSIX compliant with GUID v4 format.

Make GUID generation on POSIX compliant with GUID v4 format.

Per RFC 4122 section 4.4, the format of GUID version 4 must be
xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx, where y is one of [8, 9, A, B].
This CL updates base::GenerateGUID() on POSIX to be compliant with the
GUID v4 format.

This fix is proposed by Alex Vakulenko <avakulenko@chromium.org>;

BUG=None
TEST=base_unittests

Committed: https://crrev.com/87119864277bb405bf1303d98691f5acd5add403
Cr-Commit-Position: refs/heads/master@{#293518}

[Ben Chan, 2014-09-05 07:07:27]

benchan@chromium.org changed reviewers:
+ cmasone@chromium.org, mark@chromium.org

[Mark Mentovai, 2014-09-05 12:43:59]

LGTM

[Ben Chan, 2014-09-05 13:01:20]

The CQ bit was checked by benchan@chromium.org

[commit-bot: I haz the power, 2014-09-05 13:02:49]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/benchan@chromium.org/541633005/1

[commit-bot: I haz the power, 2014-09-05 14:29:21]

Committed patchset #1 (id:1) as 2bd522accb1d13fd8de22012b493b669f236c4c1

[commit-bot: I haz the power, 2014-09-10 03:39:30]

Patchset 1 (id:??) landed as
https://crrev.com/87119864277bb405bf1303d98691f5acd5add403
Cr-Commit-Position: refs/heads/master@{#293518}