[heap] Annotate accesses to used instance size field of a map as atomic.

This fixes false positive data race reported by TSAN.

Bug: chromium:790004
Change-Id: I6335af1735fe9ea77a26cc9919ec4f089b004cdf
Reviewed-on: https://chromium-review.googlesource.com/800936Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49766}

src/objects-inl.h

@@ -3149,14 +3149,14 @@ int Map::UnusedPropertyFields() const {
 }
 
 int Map::used_or_unused_instance_size_in_words() const {
-  return READ_BYTE_FIELD(this, kUsedOrUnusedInstanceSizeInWordsOffset);
+  return RELAXED_READ_BYTE_FIELD(this, kUsedOrUnusedInstanceSizeInWordsOffset);
 }
 
 void Map::set_used_or_unused_instance_size_in_words(int value) {
   DCHECK_LE(0, value);
   DCHECK_LE(value, 255);
-  WRITE_BYTE_FIELD(this, kUsedOrUnusedInstanceSizeInWordsOffset,
-                   static_cast<byte>(value));
+  RELAXED_WRITE_BYTE_FIELD(this, kUsedOrUnusedInstanceSizeInWordsOffset,
+                           static_cast<byte>(value));
 }
 
 int Map::UsedInstanceSize() const {