private iterator to visit all resource cache entries

src/core/SkCachedData.h

Index: src/core/SkCachedData.h
diff --git a/src/core/SkCachedData.h b/src/core/SkCachedData.h
index a8611572987b6c96721686f14cc63833e0f446d9..d44fbc58951da0602cc8e58e6cbf64b4329480a5 100644
--- a/src/core/SkCachedData.h
+++ b/src/core/SkCachedData.h
@@ -31,6 +31,10 @@ public:
     bool testing_only_isLocked() const { return fIsLocked; }
     bool testing_only_isInCache() const { return fInCache; }
 
+    SkDiscardableMemory* diagnostic_only_getDiscardable() const {
+        return kDiscardableMemory_StorageType == fStorageType ? fStorage.fDM : nullptr;
+    }
+
 protected:
     // called when fData changes. could be NULL.
     virtual void onDataChange(void* oldData, void* newData) {}