Add 6 more Linux GCE test buildslaves
Add 6 more Linux GCE test buildslaves

BUG=skia:5085

Review URL: https://codereview.chromium.org/1805943002

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