DM: make GPU tasks multithreaded again. Big refactor.

dm/DMGpuGMTask.h

-#ifndef DMGpuTask_DEFINED
-#define DMGpuTask_DEFINED
+#ifndef DMGpuGMTask_DEFINED
+#define DMGpuGMTask_DEFINED
 
 #include "DMExpectations.h"
 #include "DMReporter.h"
 #include "DMTask.h"
 #include "DMTaskRunner.h"
 #include "GrContextFactory.h"
 #include "SkBitmap.h"
 #include "SkString.h"
 #include "SkTemplates.h"
 #include "gm.h"
 
 // This is the main entry point for drawing GMs with the GPU.
 
 namespace DM {
 
-class GpuTask : public Task {
+class GpuGMTask : public GpuTask {
 public:
-    GpuTask(const char* config,
-            Reporter*,
-            TaskRunner*,
-            const Expectations&,
-            skiagm::GMRegistry::Factory,
-            GrContextFactory::GLContextType,
-            int sampleCount);
+    GpuGMTask(const char* config,
+              Reporter*,
+              TaskRunner*,
+              const Expectations&,
+              skiagm::GMRegistry::Factory,
+              GrContextFactory::GLContextType,
+              int sampleCount);
 
-    virtual void draw() SK_OVERRIDE;
-    virtual bool usesGpu() const SK_OVERRIDE { return true; }
+    virtual void draw(GrContextFactory*) SK_OVERRIDE;
     virtual bool shouldSkip() const SK_OVERRIDE;
     virtual SkString name() const SK_OVERRIDE { return fName; }
 
 private:
     SkAutoTDelete<skiagm::GM> fGM;
     const SkString fName;
     const Expectations& fExpectations;
     const GrContextFactory::GLContextType fContextType;
     const int fSampleCount;
 };
 
 }  // namespace DM
 
-#endif  // DMGpuTask_DEFINED
+#endif  // DMGpuGMTask_DEFINED