private iterator to visit all resource cache entries

include/core/SkPixelRef.h

Index: include/core/SkPixelRef.h
diff --git a/include/core/SkPixelRef.h b/include/core/SkPixelRef.h
index 0ed3099126de3481c384996cc9c55cd0d0306891..d14a0c07858b4d06feef7cd7c434c31081d9731f 100644
--- a/include/core/SkPixelRef.h
+++ b/include/core/SkPixelRef.h
@@ -24,6 +24,7 @@ class SkData;
 struct SkIRect;
 
 class GrTexture;
+class SkDiscardableMemory;
 
 /** \class SkPixelRef
 
@@ -263,6 +264,8 @@ public:
         fAddedToCache.store(true);
     }
 
+    virtual SkDiscardableMemory* diagnostic_only_getDiscardable() const { return NULL; }
+
 protected:
     /**
      *  On success, returns true and fills out the LockRec for the pixels. On