DM: write failed comparison mode .pngs one more level deep in the tree.

dm/DMTask.h

 #ifndef DMTask_DEFINED
 #define DMTask_DEFINED
 
 #include "DMReporter.h"
 #include "SkRunnable.h"
 #include "SkThreadPool.h"
 
 // DM will run() these tasks on one of two threadpools, depending on the result
 // of usesGpu().  The subclasses can call fail() to mark this task as failed,
 // or make any number of spawnChild() calls to kick off dependent tasks.
 //
 // Task deletes itself when run.
 
 namespace DM {
 
 class TaskRunner;
 
 class Task : public SkRunnable {
 public:
     Task(Reporter* reporter, TaskRunner* taskRunner);
-    Task(const Task& that);
+    Task(const Task& parent);
     virtual ~Task();
 
     void run() SK_OVERRIDE;
 
     virtual void draw() = 0;
     virtual bool usesGpu() const = 0;
     virtual bool shouldSkip() const = 0;
     virtual SkString name() const = 0;
 
+    // Returns the number of parents above this task.
+    // Top-level tasks return 0, their children 1, and so on.
+    int depth() const { return fDepth; }
+
 protected:
     void spawnChild(Task* task);
     void fail();
 
 private:
     // Both unowned.
     Reporter* fReporter;
     TaskRunner* fTaskRunner;
+    int fDepth;
 
     typedef SkRunnable INHERITED;
 };
 
 }  // namespace DM
 
 #endif  // DMTask_DEFINED