Revert of SkThreadPool ~~> SkTaskGroup

dm/DMTaskRunner.h

 #ifndef DMTaskRunner_DEFINED
 #define DMTaskRunner_DEFINED
 
 #include "DMGpuSupport.h"
-#include "SkTDArray.h"
-#include "SkTaskGroup.h"
+#include "SkThreadPool.h"
 #include "SkTypes.h"
 
+// TaskRunner runs Tasks on one of two threadpools depending on the need for a GrContextFactory.
+// It's typically a good idea to run fewer GPU threads than CPU threads (go nuts with those).
+
 namespace DM {
 
 class CpuTask;
 class GpuTask;
 
 class TaskRunner : SkNoncopyable {
 public:
-    TaskRunner() {}
+    explicit TaskRunner(int cpuThreads, int gpuThreads);
 
     void add(CpuTask* task);
+    void addNext(CpuTask* task);
     void add(GpuTask* task);
     void wait();
 
 private:
-    SkTaskGroup fCpuWork;
-    SkTDArray<GpuTask*> fGpuWork;
+    SkTThreadPool<void> fCpu;
+    SkTThreadPool<GrContextFactory> fGpu;
 };
 
 }  // namespace DM
 
 #endif  // DMTaskRunner_DEFINED