Reuse empty StoreBufferBlocks.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index f92fe62e2efaa4b828dda6bb2192a581f5e87bf6..2ad8e1fed97c1af36767652143d218d8105835c9 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -526,14 +526,15 @@ void Scavenger::IterateStoreBuffers(Isolate* isolate,
     MSAN_UNPOISON(pending, sizeof(*pending));
     intptr_t count = pending->Count();
     total_count += count;
-    for (intptr_t i = 0; i < count; i++) {
-      RawObject* raw_object = pending->At(i);
+    while (!pending->IsEmpty()) {
+      RawObject* raw_object = pending->Pop();
       ASSERT(raw_object->IsRemembered());
       raw_object->ClearRememberedBit();
       visitor->VisitingOldObject(raw_object);
       raw_object->VisitPointers(visitor);
     }
-    delete pending;
+    pending->Reset();
+    isolate->store_buffer()->PushBlock(pending);
     pending = next;
   }
   heap_->RecordData(kStoreBufferEntries, total_count);