Fix crash when querying a runtime config that is defined in environment

Fix crash when querying a runtime config that is defined in environment

Fix crash when querying a runtime config in case when there's no
skia.conf file and the config variable is still defined in the
environment.

Runs the added SkRTConf::UnitTest test as part of new "UnitTest" test.

BUG=skia:1494
Committed: http://code.google.com/p/skia/source/detail?r=10654

[Kimmo Kinnunen, 2013-08-09 08:27:47]

r? Brian as you reviewed the original commit

[bsalomon, 2013-08-09 13:14:33]

lgtm. Greg, adding you to list in case you want to do an after-the-fact review.

[commit-bot: I haz the power, 2013-08-09 13:14:40]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/kkinnunen@nvidia.com/22650007/4001

[commit-bot: I haz the power, 2013-08-09 13:23:58]

Change committed as 10654

[robertphillips, 2013-08-12 12:07:22]

This was reverted in 10669 due to Chromium Windows failures:

win:
http://build.chromium.org/p/tryserver.chromium/builders/win/builds/100045/ste...
win_layout:
http://build.chromium.org/p/tryserver.chromium/builders/win_layout/builds/790...
win_gpu:
http://build.chromium.org/p/tryserver.chromium/builders/win_gpu/builds/1742/s...

[254/3668] CXX obj\third_party\skia\src\core\skia.skpicture.obj
FAILED: ninja -t msvc -e environment.x86 -- E:\b\build\goma\gomacc.exe
"c:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\cl.exe" /nologo
/showIncludes /FC @obj\third_party\skia\src\utils\skia.skrtconf.obj.rsp /c
..\..\third_party\skia\src\utils\SkRTConf.cpp
/Foobj\third_party\skia\src\utils\skia.skrtconf.obj /Fdobj\skia\skia.pdb 
e:\b\build\slave\win_layout\build\src\third_party\skia\src\utils\skrtconf.cpp(309)
: error C3861: 'setenv': identifier not found
ninja: build stopped: subcommand failed.