Fixes bug in calculation of memory used by new space

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 819652674f2e0052d4de6b744dc69cce1300c9b7..0721fe2f91615dbc0fc9701028de5d05aec93ef9 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -201,9 +201,7 @@ class Scavenger {
     end_ = value;
   }
 
-  int64_t UsedInWords() const {
-    return (top_ - FirstObjectStart()) >> kWordSizeLog2;
-  }
+  int64_t UsedInWords() const;
   int64_t CapacityInWords() const { return to_->size_in_words(); }
   int64_t ExternalInWords() const { return external_size_ >> kWordSizeLog2; }
   SpaceUsage GetCurrentUsage() const {