Do not leak when iterator outlives disk cache backend.

net/disk_cache/blockfile/rankings.h

Index: net/disk_cache/blockfile/rankings.h
diff --git a/net/disk_cache/blockfile/rankings.h b/net/disk_cache/blockfile/rankings.h
index 4224d0058eb0fff00c12345bf82adc88cf78ad47..b523932498dab92544020161a190bf39a922e6de 100644
--- a/net/disk_cache/blockfile/rankings.h
+++ b/net/disk_cache/blockfile/rankings.h
@@ -94,8 +94,8 @@ class Rankings {
   // If we have multiple lists, we have to iterate through all at the same time.
   // This structure keeps track of where we are on the iteration.
   struct Iterator {
-    explicit Iterator(Rankings* rankings);
-    ~Iterator();
+    Iterator();
+    void Reset();
 
     List list;                     // Which entry was returned to the user.
     CacheRankingsBlock* nodes[3];  // Nodes on the first three lists.