private iterator to visit all resource cache entries

bench/ImageCacheBench.cpp

Index: bench/ImageCacheBench.cpp
diff --git a/bench/ImageCacheBench.cpp b/bench/ImageCacheBench.cpp
index 4a068b03abb064a8a76cca877c7bd8955a7bde5a..96c64f84c13454c51ba99a0e96c7f8097d78df0c 100644
--- a/bench/ImageCacheBench.cpp
+++ b/bench/ImageCacheBench.cpp
@@ -26,6 +26,8 @@ struct TestRec : public SkResourceCache::Rec {
 
     const Key& getKey() const override { return fKey; }
     size_t bytesUsed() const override { return sizeof(fKey) + sizeof(fValue); }
+    const char* getCategory() const override { return "imagecachebench-test"; }
+    SkDiscardableMemory* diagnostic_only_getDiscardable() const override { return NULL; }
 
     static bool Visitor(const SkResourceCache::Rec&, void*) {
         return true;