private iterator to visit all resource cache entries

src/lazy/SkDiscardablePixelRef.h

Index: src/lazy/SkDiscardablePixelRef.h
diff --git a/src/lazy/SkDiscardablePixelRef.h b/src/lazy/SkDiscardablePixelRef.h
index 6f422e31db675c85ff4cecfe0b8f9535dec3344b..e8451ee26905de6bd3cfb276b5a1ad4fb0dd4070 100644
--- a/src/lazy/SkDiscardablePixelRef.h
+++ b/src/lazy/SkDiscardablePixelRef.h
@@ -21,6 +21,9 @@
 class SkDiscardablePixelRef : public SkPixelRef {
 public:
     
+    SkDiscardableMemory* diagnostic_only_getDiscardable() const override {
+        return fDiscardableMemory;
+    }
 
 protected:
     ~SkDiscardablePixelRef();