Add Metrics for heap high water marks

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index ab99e07c6cf29b7385de04e7edd3882e4ce0081d..39d34782baff81a4487e710be31c5551cc275077 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -47,9 +47,9 @@ Heap::Heap(Isolate* isolate,
            intptr_t max_new_gen_semi_words,
            intptr_t max_old_gen_words,
            intptr_t max_external_words)
-    : new_space_(this, max_new_gen_semi_words, kNewObjectAlignmentOffset),
+    : isolate_(isolate),
+      new_space_(this, max_new_gen_semi_words, kNewObjectAlignmentOffset),
       old_space_(this, max_old_gen_words, max_external_words),
-      isolate_(isolate),
       read_only_(false),
       gc_in_progress_(false),
       pretenure_policy_(0) {