DM: make GPU tasks multithreaded again. Big refactor.

dm/DMExpectationsTask.h

Index: dm/DMExpectationsTask.h
diff --git a/dm/DMExpectationsTask.h b/dm/DMExpectationsTask.h
index cf76fc8bdf2b61b35958df854031577269383135..7000de4b68e46ef7381f73eca105a24be338eca8 100644
--- a/dm/DMExpectationsTask.h
+++ b/dm/DMExpectationsTask.h
@@ -10,12 +10,11 @@ namespace DM {
 
 // ExpectationsTask compares an SkBitmap against some Expectations.
 // Moving this off the GPU threadpool is a nice (~30%) runtime win.
-class ExpectationsTask : public Task {
+class ExpectationsTask : public CpuTask {
 public:
     ExpectationsTask(const Task& parent, const Expectations&, SkBitmap);
 
     virtual void draw() SK_OVERRIDE;
-    virtual bool usesGpu() const SK_OVERRIDE { return false; }
     virtual bool shouldSkip() const SK_OVERRIDE { return false; }
     virtual SkString name() const SK_OVERRIDE { return fName; }