Fixes bug in calculation of memory used by new space

Changes Scavenger to use mutator's top_ to calculate memory used in
new space instead of the Scavenger's when the VM is not scavenging.

Closes https://github.com/dart-lang/sdk/issues/30311

R=cbernaschina@google.com, rmacnak@google.com

Review-Url: https://codereview.chromium.org/2992153002 .

runtime/vm/scavenger.cc

@@ -948,4 +948,11 @@ void Scavenger::Evacuate() {
   ASSERT((UsedInWords() == 0) || failed_to_promote_);
 }
 
+int64_t Scavenger::UsedInWords() const {
+  uword saved_top = FlushTLS();
+  int64_t used_in_words = (top_ - FirstObjectStart()) >> kWordSizeLog2;
+  UnflushTLS(saved_top);
+  return used_in_words;
+}
+
 }  // namespace dart

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 {