Implement cache counting for the simple and memory backends.

net/disk_cache/simple/simple_index.h

Index: net/disk_cache/simple/simple_index.h
diff --git a/net/disk_cache/simple/simple_index.h b/net/disk_cache/simple/simple_index.h
index 11adab9d72f895377d1822cd94ec4ffb4f97c618..5a8046da82291611b66c0a85e78a4d889e43d3f6 100644
--- a/net/disk_cache/simple/simple_index.h
+++ b/net/disk_cache/simple/simple_index.h
@@ -130,6 +130,10 @@ class NET_EXPORT_PRIVATE SimpleIndex
   // Returns number of indexed entries.
   int32 GetEntryCount() const;
 
+  // Returns the size of the entire cache in bytes. Can only be called after the
+  // index has been initialized.
+  uint64 GetCacheSize() const;
+
   // Returns whether the index has been initialized yet.
   bool initialized() const { return initialized_; }