Run GM in multiple, parallel processes
Run GM in multiple, parallel processes

This should speed up the running time.
Review URL: https://codereview.appspot.com/6749060

git-svn-id: 32fc27f4dcfb6c0385cd9719852b95fe6680452d@6045 2bbb7eff-a529-9590-31e7-b0007b416f81

google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot