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

dm/DMUtil.h

 #ifndef DMUtil_DEFINED
 #define DMUtil_DEFINED
 
 #include "SkBitmap.h"
 #include "SkString.h"
 #include "gm_expectations.h"
 
 // Small free functions used in more than one place in DM.
 
 namespace DM {
 
 // UnderJoin("a", "b") -> "a_b"
 SkString UnderJoin(const char* a, const char* b);
 
-// Png("a") -> "a.png"
-SkString Png(SkString s);
-
 // Draw gm to picture.  Passes recordFlags to SkPicture::beginRecording().
 void RecordPicture(skiagm::GM* gm, SkPicture* picture, uint32_t recordFlags = 0);
 
 // Prepare bitmap to have gm draw into it with this config.
 void SetupBitmap(const SkBitmap::Config config, skiagm::GM* gm, SkBitmap* bitmap);
 
 // Draw picture to bitmap.
 void DrawPicture(SkPicture* picture, SkBitmap* bitmap);
 
 // Are these identical bitmaps?
 bool BitmapsEqual(const SkBitmap& a, const SkBitmap& b);
 
 }  // namespace DM
 
 #endif  // DMUtil_DEFINED