extensions: Cleanup class/struct fwd declarations

extensions/browser/value_store/leveldb_value_store.h

Index: extensions/browser/value_store/leveldb_value_store.h
diff --git a/extensions/browser/value_store/leveldb_value_store.h b/extensions/browser/value_store/leveldb_value_store.h
index a9f5b59d0090b7fbb287e42e4e771807f161e57d..4a55b8912577b4e3a80203dec45caa089148c479 100644
--- a/extensions/browser/value_store/leveldb_value_store.h
+++ b/extensions/browser/value_store/leveldb_value_store.h
@@ -19,10 +19,6 @@
 #include "extensions/browser/value_store/value_store.h"
 #include "third_party/leveldatabase/src/include/leveldb/db.h"
 
-namespace base {
-class HistogramBase;
-}  // namespace base
-
 // Value store area, backed by a leveldb database.
 // All methods must be run on the FILE thread.
 class LeveldbValueStore : public ValueStore,