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 6493a75e0b6355943eefae86176b57f86e598325..71cfb8aa2fb86e20c9f2c86c2da04e983dbbd2c9 100644
--- a/src/core/SkCachedData.h
+++ b/src/core/SkCachedData.h
@@ -30,6 +30,10 @@ 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) {}