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>
diff --git a/infra/bots/recipes/perf.py b/infra/bots/recipes/perf.py
index b330fdc..96a2a3b 100644
--- a/infra/bots/recipes/perf.py
+++ b/infra/bots/recipes/perf.py
@@ -126,6 +126,11 @@
   args.append('--config')
   args.extend(configs)
 
+  # By default, we test with GPU threading enabled. Leave PixelC devices
+  # running without threads, just to get some coverage of that code path.
+  if 'PixelC' in bot:
+    args.extend(['--gpuThreads', '0'])
+
   if 'Valgrind' in bot:
     # Don't care about Valgrind performance.
     args.extend(['--loops',   '1'])