[heap] Minor MC: Free and prefree empty remembered set buckets

No-try: true
Bug: chromium:651354, chromium:788113
Change-Id: Iff2051751c4688c307e8f23ddd08de203974e842
Reviewed-on: https://chromium-review.googlesource.com/800272Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49751}

src/heap/mark-compact.cc

@@ -2186,6 +2186,15 @@ void MinorMarkCompactCollector::CollectGarbage() {
     }
   }
 
+  RememberedSet<OLD_TO_NEW>::IterateMemoryChunks(
+      heap(), [](MemoryChunk* chunk) {
+        if (chunk->SweepingDone()) {
+          RememberedSet<OLD_TO_NEW>::FreeEmptyBuckets(chunk);
+        } else {
+          RememberedSet<OLD_TO_NEW>::PreFreeEmptyBuckets(chunk);
+        }
+      });
+
   heap()->account_external_memory_concurrently_freed();
 }