heap: Fix BlinkGC.MainThreadMarkingThroughput

The metric is supposed to compute time spent per marked bytes and not
allocated heap size which includes garbage as well.

Bug: 986235
Change-Id: Ifdc1eedb276f8ce6a324774e233c132fefa1c747
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087726
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747207}

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

@@ -954,14 +954,13 @@ void UpdateHistograms(const ThreadHeapStatsCollector::Event& event) {
       event.scope_data[ThreadHeapStatsCollector::kMarkWeakProcessing]);
 
   base::TimeDelta marking_duration = event.foreground_marking_time();
-  constexpr size_t kMinObjectSizeForReportingThroughput = 1024 * 1024;
+  constexpr size_t kMinMarkedBytesForReportingThroughput = 1024 * 1024;
   if (base::TimeTicks::IsHighResolution() &&
-      (event.object_size_in_bytes_before_sweeping >
-       kMinObjectSizeForReportingThroughput) &&
+      (event.marked_bytes > kMinMarkedBytesForReportingThroughput) &&
       !marking_duration.is_zero()) {
     DCHECK_GT(marking_duration.InMillisecondsF(), 0.0);
     const int main_thread_marking_throughput_mb_per_s = static_cast<int>(
-        static_cast<double>(event.object_size_in_bytes_before_sweeping) /
+        static_cast<double>(event.marked_bytes) /
         marking_duration.InMillisecondsF() * 1000 / 1024 / 1024);
     UMA_HISTOGRAM_COUNTS_100000("BlinkGC.MainThreadMarkingThroughput",
                                 main_thread_marking_throughput_mb_per_s);