Correctly recognize emptiness of IDMap.

base/id_map.h

Index: base/id_map.h
diff --git a/base/id_map.h b/base/id_map.h
index b108dbd8755f706c49cecd555cac7815e0bf8ba7..acfba42bb94a61b1beeeeb128f2a7abe9090a603 100644
--- a/base/id_map.h
+++ b/base/id_map.h
@@ -82,7 +82,7 @@ class IDMap {
   }
 
   bool IsEmpty() const {
-    return data_.empty();
+    return size() == 0u;
   }
 
   T* Lookup(KeyType id) const {
@@ -93,7 +93,7 @@ class IDMap {
   }
 
   size_t size() const {
-    return data_.size();
+    return data_.size() - removed_ids_.size();
   }
 
   // It is safe to remove elements from the map during iteration. All iterators