Implement cache counting for the simple and memory backends.

net/disk_cache/simple/simple_index.cc

Index: net/disk_cache/simple/simple_index.cc
diff --git a/net/disk_cache/simple/simple_index.cc b/net/disk_cache/simple/simple_index.cc
index fd01abd89946aa2c455c50865b96adbae50c9a10..555780119d3f147341c4cb40a8bd600f0ca42d21 100644
--- a/net/disk_cache/simple/simple_index.cc
+++ b/net/disk_cache/simple/simple_index.cc
@@ -239,6 +239,11 @@ int32 SimpleIndex::GetEntryCount() const {
   return entries_set_.size();
 }
 
+uint64 SimpleIndex::GetCacheSize() const {
+  DCHECK(initialized_);
+  return cache_size_;
+}
+
 void SimpleIndex::Insert(uint64 entry_hash) {
   DCHECK(io_thread_checker_.CalledOnValidThread());
   // Upon insert we don't know yet the size of the entry.