GTTF: Fix handling of PRE_ tests

- skip dependent tests if a PRE_ test failed
- simplify ParallelTestLauncher code now that we don't need sequencing

BUG=236893
R=jam@chromium.org

Review URL: https://codereview.chromium.org/24892002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225741 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225741