Inform disk cache of WebKit memory cache hits.

net/disk_cache/disk_cache.h

Index: net/disk_cache/disk_cache.h
diff --git a/net/disk_cache/disk_cache.h b/net/disk_cache/disk_cache.h
index 973b6e61a507b1be1d97707eb744ef8a33dc2366..dc320ccbbd54fefa18f00975baa1e12962eb2ef2 100644
--- a/net/disk_cache/disk_cache.h
+++ b/net/disk_cache/disk_cache.h
@@ -137,6 +137,10 @@ class NET_API Backend {
   // Return a list of cache statistics.
   virtual void GetStats(
       std::vector<std::pair<std::string, std::string> >* stats) = 0;
+
+  // Called whenever an external cache in the system reuses the resource
+  // referred to by |key|.
+  virtual void OnExternalCacheHit(const std::string& key) = 0;
 };
 
 // This interface represents an entry in the disk cache.