fix typo in SkTaskGroup.h

This was meant to stay -1 (num_cores() threads) not to become 1 (1 thread).

Change-Id: Ib9b8e1963c0cd16d4c0aba02ccf7a65890a5ef78
Reviewed-on: https://skia-review.googlesource.com/8851
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/core/SkTaskGroup.h

@@ -32,7 +32,7 @@ public:
     // A convenience for testing tools.
     // Creates and owns a thread pool, and passes it to SkExecutor::SetDefault().
     struct Enabler {
-        explicit Enabler(int threads = 1);  // -1 -> num_cores, 0 -> noop
+        explicit Enabler(int threads = -1);  // -1 -> num_cores, 0 -> noop
         std::unique_ptr<SkExecutor> fThreadPool;
     };