Generalize the interface of page space controller.

runtime/vm/spaces.h

Index: runtime/vm/spaces.h
===================================================================
--- runtime/vm/spaces.h	(revision 0)
+++ runtime/vm/spaces.h	(revision 0)
@@ -0,0 +1,26 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#ifndef VM_SPACES_H_
+#define VM_SPACES_H_
+
+// This file contains utilities shared by old and new space.
+// TODO(koda): Create Space base class with Space::CurrentUsage().
+
+namespace dart {
+
+// Usage statistics for a space/generation at a particular moment in time.
+struct SpaceUsage {
+  SpaceUsage()
+    : capacity_in_words(0),
+      used_in_words(0),
+      external_in_words(0) {}
+  intptr_t capacity_in_words;
+  intptr_t used_in_words;
+  intptr_t external_in_words;
+};
+
+}  // namespace dart
+
+#endif  // VM_SPACES_H_