Let DM run unit tests.

dm/DMTaskRunner.h

 #ifndef DMTaskRunner_DEFINED
 #define DMTaskRunner_DEFINED
 
+#include "GrContextFactory.h"
 #include "SkThreadPool.h"
 #include "SkTypes.h"
 
-// TaskRunner runs Tasks on one of two threadpools depending on the Task's usesGpu() method.
-// This lets us drive the GPU with a small number of threads (e.g. 2 or 4 can be faster than 1)
-// while not swamping it with requests from the full fleet of threads that CPU-bound tasks run on.
+// TaskRunner runs Tasks on one of two threadpools depending on the Task's usesGpu() method.  This
+// lets us drive the GPU from a single thread while parallelizing CPU-bound work.
 
 namespace DM {
 
 class Task;
 
 class TaskRunner : SkNoncopyable {
 public:
-    TaskRunner(int cputhreads, int gpuThreads);
+    explicit TaskRunner(int cputhreads);
 
     void add(Task* task);
     void wait();
 
+    // This can only be safely called from a GPU task's draw() method.
+    GrContextFactory* getGrContextFactory() const { return fGrContextFactory; }
+
 private:
     SkThreadPool fMain, fGpu;
+    GrContextFactory* fGrContextFactory;  // Created and destroyed on fGpu threadpool.
 };
 
 }  // namespace DM
 
 #endif  // DMTaskRunner_DEFINED