Turn on GPU threading by default to get better test/perf coverage

Explicitly disable it on PixelC. This is arbitrary, so we continue to
get coverage of the single-threaded code.

Bug: skia:
Change-Id: I0ac91f7ca58652933db452720f353068cf2d0f2d
Reviewed-on: https://skia-review.googlesource.com/50000
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed