Add Metrics for heap high water marks

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index a62e27ea8d2b2fc7b17f911b28b7fd44f230c817..496a9c8ef049af3ffc2956ddc883a4a1a9536a60 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -311,12 +311,12 @@ class Heap {
   // of this heap.
   void GetMergedAddressRange(uword* start, uword* end) const;
 
+  Isolate* isolate_;
+
   // The different spaces used for allocation.
   Scavenger new_space_;
   PageSpace old_space_;
 
-  Isolate* isolate_;
-
   WeakTable* new_weak_tables_[kNumWeakSelectors];
   WeakTable* old_weak_tables_[kNumWeakSelectors];