DM: make GPU tasks multithreaded again. Big refactor.

dm/DMPipeTask.h

 #ifndef DMPipeTask_DEFINED
 #define DMPipeTask_DEFINED
 
 #include "DMTask.h"
 #include "SkBitmap.h"
 #include "SkString.h"
 #include "SkTemplates.h"
 #include "gm.h"
 
 // Sends a GM through a pipe, draws it, and compares against the reference bitmap.
 
 namespace DM {
 
-class PipeTask : public Task {
+class PipeTask : public CpuTask {
 
 public:
     PipeTask(const Task& parent,        // PipeTask must be a child task.  Pass its parent here.
              skiagm::GM*,               // GM to run through a pipe.  Takes ownership.
              SkBitmap reference,        // Bitmap to compare pipe results to.
              bool crossProcess,         // Should we set up a cross process pipe?
              bool sharedAddressSpace);  // If cross process, should it assume shared address space?
 
     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 uint32_t fFlags;
     const SkString fName;
     SkAutoTDelete<skiagm::GM> fGM;
     const SkBitmap fReference;
 };
 
 }  // namespace DM
 
 #endif  // DMPipeTask_DEFINED