[gce] Switch to using DefaultTokenSource which obviates the need for a client_secrets json file.
[gce] Switch to using DefaultTokenSource which obviates the need for a client_secrets json file.

Bug: skia:
Change-Id: If7ed558be7cdac23f028b4f833f59e6b8177861b
Reviewed-on: https://skia-review.googlesource.com/128120
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>

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