DM: make GPU tasks multithreaded again. Big refactor.

dm/DMWriteTask.h

 #ifndef DMWriteTask_DEFINED
 #define DMWriteTask_DEFINED
 
 #include "DMExpectations.h"
 #include "DMTask.h"
 #include "SkBitmap.h"
 #include "SkString.h"
 #include "SkTArray.h"
 
 
 // Writes a bitmap to a file.
 
 namespace DM {
 
-class WriteTask : public Task {
+class WriteTask : public CpuTask {
 
 public:
     WriteTask(const Task& parent,  // WriteTask must be a child Task.  Pass its parent here.
               SkBitmap bitmap);    // Bitmap to write.
 
     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;
 
     // Reads image files WriteTask wrote under root and compares them with bitmap.
     class Expectations : public DM::Expectations {
     public:
         explicit Expectations(const char* root) : fRoot(root) {}
 
         bool check(const Task& task, SkBitmap bitmap) const SK_OVERRIDE;
     private:
         const char* fRoot;
     };
 
 private:
     SkTArray<SkString> fSuffixes;
     SkString fGmName;
     const SkBitmap fBitmap;
 
     void makeDirOrFail(SkString dir);
 };
 
 }  // namespace DM
 
 #endif  // DMWriteTask_DEFINED