heap: Resolve data race when getting background marking time stat

Incremental marking queries the background_marking_time statistic when
it evaluates progress. The concurrent marker can update the statistic
at the same time. Querying the statistic needs to be atomic.

Bug: 986235
Change-Id: If69dca913f9ba2dba2d03a27801443248ce8f272
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893266
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711423}

third_party/blink/renderer/platform/heap/heap_stats_collector.cc

@@ -196,7 +196,7 @@ base::TimeDelta ThreadHeapStatsCollector::Event::foreground_marking_time()
 base::TimeDelta ThreadHeapStatsCollector::Event::background_marking_time()
     const {
   return base::TimeDelta::FromMicroseconds(
-      concurrent_scope_data[kConcurrentMark]);
+      base::subtle::NoBarrier_Load(&concurrent_scope_data[kConcurrentMark]));
 }
 
 base::TimeDelta ThreadHeapStatsCollector::Event::marking_time() const {