Add Heap::isolate_ to simplify code.

runtime/vm/pages.h

Index: runtime/vm/pages.h
===================================================================
--- runtime/vm/pages.h	(revision 35410)
+++ runtime/vm/pages.h	(working copy)
@@ -114,7 +114,10 @@
 // PageSpaceController controls the heap size.
 class PageSpaceController {
  public:
-  PageSpaceController(int heap_growth_ratio,
+  // The heap is passed in for recording stats only. The controller does not
+  // invoke GC by itself.
+  PageSpaceController(Heap* heap,
+                      int heap_growth_ratio,
                       int heap_growth_max,
                       int garbage_collection_time_ratio);
   ~PageSpaceController();
@@ -146,6 +149,8 @@
   }
 
  private:
+  Heap* heap_;
+
   bool is_enabled_;
 
   // Usage after last evaluated GC or last enabled.