Implement cache counting for the simple and memory backends.

net/disk_cache/simple/simple_backend_impl.h

Index: net/disk_cache/simple/simple_backend_impl.h
diff --git a/net/disk_cache/simple/simple_backend_impl.h b/net/disk_cache/simple/simple_backend_impl.h
index c9debd0acc1f1912bae892e84227b9df3e28b0f6..c8cac21124e634a0684a2dfbb4c35627c751b435 100644
--- a/net/disk_cache/simple/simple_backend_impl.h
+++ b/net/disk_cache/simple/simple_backend_impl.h
@@ -140,6 +140,10 @@ class NET_EXPORT_PRIVATE SimpleBackendImpl : public Backend,
                          const CompletionCallback& callback,
                          int result);
 
+  // Calculates the size of the entire cache. Invoked when the index is ready.
+  void IndexReadyForSizeCalculation(const CompletionCallback& callback,
+                                    int result);
+
   // Try to create the directory if it doesn't exist. This must run on the IO
   // thread.
   static DiskStatResult InitCacheStructureOnDisk(const base::FilePath& path,