DM: Add support for reading a directory of images with --expectations (-r).

dm/DMChecksumTask.h

-#ifndef DMChecksumTask_DEFINED
-#define DMChecksumTask_DEFINED
-
-#include "DMTask.h"
-#include "SkBitmap.h"
-#include "SkString.h"
-#include "gm_expectations.h"
-
-namespace DM {
-
-// ChecksumTask compares an SkBitmap against some Expectations.
-// Moving this off the GPU threadpool is a nice (~30%) runtime win.
-class ChecksumTask : public Task {
-public:
-    ChecksumTask(const Task& parent, skiagm::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; }
-
-private:
-    const SkString fName;
-    const skiagm::Expectations fExpectations;
-    const SkBitmap fBitmap;
-};
-
-}  // namespace DM
-
-#endif  // DMChecksumTask_DEFINED