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
R=bsalomon@google.com, humper@google.com
Author: kkinnunen@nvidia.com
Review URL: https://chromiumcodereview.appspot.com/
22650007
git-svn-id: http://skia.googlecode.com/svn/trunk@10654
2bbb7eff-a529-9590-31e7-
b0007b416f81