Revert 233725 "GTTF: Make test launcher unset GTEST_ env variabl..."

Broke all of the Windows XP bots

> GTTF: Make test launcher unset GTEST_ env variables since it handles all of sharding.
> 
> BUG=236893
> R=sky@chromium.org
> 
> Review URL: https://codereview.chromium.org/60983004

TBR=phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/66043005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233762 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233762