Fix compile errors on 64 bit platforms.

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 11265)
+++ src/heap.h	(working copy)
@@ -1326,7 +1326,8 @@
 
   // Adjusts the amount of registered external memory.
   // Returns the adjusted value.
-  inline int AdjustAmountOfExternalAllocatedMemory(int change_in_bytes);
+  inline intptr_t AdjustAmountOfExternalAllocatedMemory(
+      intptr_t change_in_bytes);
 
   // Allocate uninitialized fixed array.
   MUST_USE_RESULT MaybeObject* AllocateRawFixedArray(int length);
@@ -1646,7 +1647,7 @@
   int gc_post_processing_depth_;
 
   // Returns the amount of external memory registered since last global gc.
-  int PromotedExternalMemorySize();
+  intptr_t PromotedExternalMemorySize();
 
   int ms_count_;  // how many mark-sweep collections happened
   unsigned int gc_count_;  // how many gc happened
@@ -1714,7 +1715,7 @@
   intptr_t amount_of_external_allocated_memory_;
 
   // Caches the amount of external memory registered at the last global gc.
-  int amount_of_external_allocated_memory_at_last_global_gc_;
+  intptr_t amount_of_external_allocated_memory_at_last_global_gc_;
 
   // Indicates that an allocation has failed in the old generation since the
   // last GC.