GTTF: Print exit code from the subprocess for better debugging.
GTTF: Print exit code from the subprocess for better debugging.

This info is critical, since the exit code ultimately determines
test success or failure.

BUG=297176
R=stip@chromium.org

Review URL: https://codereview.chromium.org/25641003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@227046 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build