Restore a really single-threaded mode to DM.

src/utils/SkTaskGroup.h

Index: src/utils/SkTaskGroup.h
diff --git a/src/utils/SkTaskGroup.h b/src/utils/SkTaskGroup.h
index d9e6f597d05b173071acc1254d7045e553431fba..dddd4ad442ecb03041f2578c688e8ef8060f43dd 100644
--- a/src/utils/SkTaskGroup.h
+++ b/src/utils/SkTaskGroup.h
@@ -15,7 +15,7 @@ class SkTaskGroup : SkNoncopyable {
 public:
     // Create one of these in main() to enable SkTaskGroups globally.
     struct Enabler : SkNoncopyable {
-        explicit Enabler(int threads = 0);  // Default is system-reported core count.
+        explicit Enabler(int threads = -1);  // Default is system-reported core count.
         ~Enabler();
     };