GTTF: Update --help for new test launcher.

Note that the switches are a little bit different between unit and browser
test launchers.

BUG=322266, 236893
R=sky@chromium.org

Review URL: https://codereview.chromium.org/103413003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238867 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238867