Re-add '--gpuThreads 0' tests.

Bug: skia:7394
Change-Id: I2e7ed586d81d19a0b7895ff81ce6670eb7cc5003
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/83445
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
5 files changed