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/tools/flags/SkCommonFlags.cpp b/tools/flags/SkCommonFlags.cpp
index c097f84..0fc1e1d 100644
--- a/tools/flags/SkCommonFlags.cpp
+++ b/tools/flags/SkCommonFlags.cpp
@@ -56,8 +56,8 @@
 DEFINE_int32_2(threads, j, -1, "Run threadsafe tests on a threadpool with this many extra threads, "
                                "defaulting to one extra thread per core.");
 
-DEFINE_int32(gpuThreads, 0, "Create this many extra threads to assist with GPU work, "
-                            "including software path rendering.");
+DEFINE_int32(gpuThreads, 2, "Create this many extra threads to assist with GPU work, "
+                            "including software path rendering. Defaults to two.");
 
 DEFINE_bool2(verbose, v, false, "enable verbose output from the test driver.");