private iterator to visit all resource cache entries

src/core/SkYUVPlanesCache.cpp

Index: src/core/SkYUVPlanesCache.cpp
diff --git a/src/core/SkYUVPlanesCache.cpp b/src/core/SkYUVPlanesCache.cpp
index 79136db795efcdf05a1aa788c66474895edb1972..b113c079743f01b9e12adde16fb7f1c38e2cd31b 100644
--- a/src/core/SkYUVPlanesCache.cpp
+++ b/src/core/SkYUVPlanesCache.cpp
@@ -48,6 +48,10 @@ struct YUVPlanesRec : public SkResourceCache::Rec {
 
     const Key& getKey() const override { return fKey; }
     size_t bytesUsed() const override { return sizeof(*this) + fValue.fData->size(); }
+    const char* getCategory() const override { return "yuv-planes"; }
+    SkDiscardableMemory* diagnostic_only_getDiscardable() const override {
+        return fValue.fData->diagnostic_only_getDiscardable();
+    }
 
     static bool Visitor(const SkResourceCache::Rec& baseRec, void* contextData) {
         const YUVPlanesRec& rec = static_cast<const YUVPlanesRec&>(baseRec);