We have at least three different UUID/GUID implementations in Chromium
and they all use slightly different UUID/GUID format criteria.

From code health (and consistent behavior) pov it'd be nice
to unify them.

- base/guid_posix.cc - returns mostly-correct Ver4 GUID string but in uppercase
- base/guid_win.cc - returns mostly-correct Ver4 GUID string but in uppercase
- third_party/WebKit/Source/platform/UUID.cpp - returns correct Ver4 GUID

This patch tries to unify them into one location in base/guid.cc.

BUG=561879
TEST=base_unittests:GUID*
TEST=blink_platform_unittests:UUID*

Review URL: https://codereview.chromium.org/1906723003

Cr-Commit-Position: refs/heads/master@{#390042}