Revert 233963 "Revert 233960 "GTTF: Make test launcher unset GTE..."

> 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

TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/64623004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233964 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233964