Restore a really single-threaded mode to DM.

src/utils/SkTaskGroup.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTaskGroup_DEFINED
 #define SkTaskGroup_DEFINED
 
 #include "SkTypes.h"
 #include "SkRunnable.h"
 
 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();
     };
 
     SkTaskGroup();
     ~SkTaskGroup() { this->wait(); }
 
     // Add a task to this SkTaskGroup.  It will likely run() on another thread.
     void add(SkRunnable*);
 
     // Block until all Tasks previously add()ed to this SkTaskGroup have run().
     // You may safely reuse this SkTaskGroup after wait() returns.
     void wait();
 
 private:
     /*atomic*/ int32_t fPending;
 };
 
 #endif//SkTaskGroup_DEFINED