Add max post-gc heap usage metrics

runtime/vm/pages.h

Index: runtime/vm/pages.h
diff --git a/runtime/vm/pages.h b/runtime/vm/pages.h
index 7cc123b6ff16765859c336b1ce9cb1f6ed21a599..39ef84eff191e593f28963e18e8054b9ae0300cc 100644
--- a/runtime/vm/pages.h
+++ b/runtime/vm/pages.h
@@ -218,8 +218,8 @@ class PageSpace {
            NeedsExternalGC();
   }
 
-  intptr_t UsedInWords() const { return usage_.used_in_words; }
-  intptr_t CapacityInWords() const {
+  int64_t UsedInWords() const { return usage_.used_in_words; }
+  int64_t CapacityInWords() const {
     MutexLocker ml(pages_lock_);
     return usage_.capacity_in_words;
   }
@@ -234,8 +234,9 @@ class PageSpace {
   }
 
   void UpdateMaxCapacityLocked();
+  void UpdateMaxUsed();
 
-  intptr_t ExternalInWords() const {
+  int64_t ExternalInWords() const {
     return usage_.external_in_words;
   }
   SpaceUsage GetCurrentUsage() const {