Generalize the interface of page space controller.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
===================================================================
--- runtime/vm/scavenger.h	(revision 34735)
+++ runtime/vm/scavenger.h	(working copy)
@@ -10,6 +10,7 @@
 #include "vm/flags.h"
 #include "vm/globals.h"
 #include "vm/raw_object.h"
+#include "vm/spaces.h"
 #include "vm/virtual_memory.h"
 #include "vm/visitor.h"
 
@@ -81,6 +82,13 @@
   intptr_t ExternalInWords() const {
     return external_size_ >> kWordSizeLog2;
   }
+  SpaceUsage GetCurrentUsage() const {
+    SpaceUsage usage;
+    usage.used_in_words = UsedInWords();
+    usage.capacity_in_words = CapacityInWords();
+    usage.external_in_words = ExternalInWords();
+    return usage;
+  }
 
   void VisitObjects(ObjectVisitor* visitor) const;
   void VisitObjectPointers(ObjectPointerVisitor* visitor) const;