Add max post-gc heap usage metrics

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index 496a9c8ef049af3ffc2956ddc883a4a1a9536a60..4639c2602b5803deacf23486e9740704088fef84 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -158,9 +158,9 @@ class Heap {
   void PrintSizes() const;
 
   // Return amount of memory used and capacity in a space, excluding external.
-  intptr_t UsedInWords(Space space) const;
-  intptr_t CapacityInWords(Space space) const;
-  intptr_t ExternalInWords(Space space) const;
+  int64_t UsedInWords(Space space) const;
+  int64_t CapacityInWords(Space space) const;
+  int64_t ExternalInWords(Space space) const;
   // Return the amount of GCing in microseconds.
   int64_t GCTimeInMicros(Space space) const;
 
@@ -222,6 +222,8 @@ class Heap {
 
   bool gc_in_progress();
 
+  void UpdateGlobalMaxUsed();
+
   static bool IsAllocatableInNewSpace(intptr_t size) {
     return size <= kNewAllocatableSize;
   }