Revert 233960 "GTTF: Make test launcher unset GTEST_ env variabl..."

> GTTF: Make test launcher unset GTEST_ env variables since it handles all of sharding.
> 
> This is a relanding of https://codereview.chromium.org/60983004/ which checks
> for variable existence before unsetting to avoid failing on Windows XP.
> 
> BUG=236893
> R=kbr@chromium.org
> 
> Review URL: https://codereview.chromium.org/67183003

TBR=phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/66603005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233963 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233963