Do not leak when iterator outlives disk cache backend.

net/disk_cache/blockfile/rankings.cc

Index: net/disk_cache/blockfile/rankings.cc
diff --git a/net/disk_cache/blockfile/rankings.cc b/net/disk_cache/blockfile/rankings.cc
index 7c93b431bcbbb8fd3cb1eba85e6dfb9e69ee3ef4..17e79913b133e33e1a9fce8afe1d8b7d8e369e4a 100644
--- a/net/disk_cache/blockfile/rankings.cc
+++ b/net/disk_cache/blockfile/rankings.cc
@@ -202,14 +202,16 @@ Rankings::ScopedRankingsBlock::ScopedRankingsBlock(
     Rankings* rankings, CacheRankingsBlock* node)
     : scoped_ptr<CacheRankingsBlock>(node), rankings_(rankings) {}
 
-Rankings::Iterator::Iterator(Rankings* rankings) {
+Rankings::Iterator::Iterator() {
   memset(this, 0, sizeof(Iterator));
-  my_rankings = rankings;
 }
 
-Rankings::Iterator::~Iterator() {
-  for (int i = 0; i < 3; i++)
-    ScopedRankingsBlock(my_rankings, nodes[i]);
+void Rankings::Iterator::Reset() {
+  if (my_rankings) {
+    for (int i = 0; i < 3; i++)
+      ScopedRankingsBlock(my_rankings, nodes[i]);
+  }
+  memset(this, 0, sizeof(Iterator));
 }
 
 Rankings::Rankings() : init_(false) {}