heap: Fix BlinkGC.MainThreadMarkingThroughput

This histogram should be computed from foreground marking time only.

Bug: 986235
Change-Id: I9ca3471a7fee2547ac1a89dcae2c9515b6e173a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2074878
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@{#744684}

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

@@ -953,7 +953,7 @@ void UpdateHistograms(const ThreadHeapStatsCollector::Event& event) {
       "BlinkGC.TimeForGlobalWeakProcessing",
       event.scope_data[ThreadHeapStatsCollector::kMarkWeakProcessing]);
 
-  base::TimeDelta marking_duration = event.marking_time();
+  base::TimeDelta marking_duration = event.foreground_marking_time();
   constexpr size_t kMinObjectSizeForReportingThroughput = 1024 * 1024;
   if (base::TimeTicks::IsHighResolution() &&
       (event.object_size_in_bytes_before_sweeping >