Record the addresses of pages that are unmapped to aid

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 11059)
+++ src/heap.h	(working copy)
@@ -1583,6 +1583,9 @@
     set_construct_stub_deopt_pc_offset(Smi::FromInt(pc_offset));
   }
 
+  // For post mortem debugging.
+  void RememberUnmappedPage(Address page, bool compacted);
+
  private:
   Heap();
 
@@ -1634,6 +1637,11 @@
   int ms_count_;  // how many mark-sweep collections happened
   unsigned int gc_count_;  // how many gc happened
 
+  // For post mortem debugging.
+  static const int kRememberedUnmappedPages = 128;
+  int remembered_unmapped_pages_index_;
+  Address remembered_unmapped_pages_[kRememberedUnmappedPages];
+
   // Total length of the strings we failed to flatten since the last GC.
   int unflattened_strings_length_;
 
@@ -1781,7 +1789,6 @@
 
   inline void UpdateOldSpaceLimits();
 
-
   // Allocate an uninitialized object in map space.  The behavior is identical
   // to Heap::AllocateRaw(size_in_bytes, MAP_SPACE), except that (a) it doesn't
   // have to test the allocation space argument and (b) can reduce code size