DM: make GPU tasks multithreaded again. Big refactor.

dm/DMSerializeTask.h

 #ifndef DMSerializeTask_DEFINED
 #define DMSerializeTask_DEFINED
 
 #include "DMTask.h"
 #include "SkBitmap.h"
 #include "SkString.h"
 #include "SkTemplates.h"
 #include "gm.h"
 
 // Record a picture, serialize it, deserialize it, then draw it and compare to reference bitmap.
 
 namespace DM {
 
-class SerializeTask : public Task {
+class SerializeTask : public CpuTask {
 
 public:
     SerializeTask(const Task& parent,
                   skiagm::GM*,
                   SkBitmap reference);
 
     virtual void draw() SK_OVERRIDE;
-    virtual bool usesGpu() const SK_OVERRIDE { return false; }
     virtual bool shouldSkip() const SK_OVERRIDE;
     virtual SkString name() const SK_OVERRIDE { return fName; }
 
 private:
     const SkString fName;
     SkAutoTDelete<skiagm::GM> fGM;
     const SkBitmap fReference;
 };
 
 }  // namespace DM
 
 #endif  // DMSerializeTask_DEFINED