heap: Only emit compaction freed bytes when compacting

Avoid biasing the metric with many 0-samples and only emit the sample
when the GC was actually compacting memory.

Bug: 1056170
Change-Id: I906b7b639fa96b53acc6f3b7235a015f6322123f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2098625Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749598}

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

@@ -14,11 +14,13 @@ namespace blink {
 void ThreadHeapStatsCollector::IncreaseCompactionFreedSize(size_t bytes) {
   DCHECK(is_started_);
   current_.compaction_freed_bytes += bytes;
+  current_.compaction_recorded_events = true;
 }
 
 void ThreadHeapStatsCollector::IncreaseCompactionFreedPages(size_t pages) {
   DCHECK(is_started_);
   current_.compaction_freed_pages += pages;
+  current_.compaction_recorded_events = true;
 }
 
 void ThreadHeapStatsCollector::IncreaseAllocatedObjectSize(size_t bytes) {

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

@@ -259,6 +259,7 @@ class PLATFORM_EXPORT ThreadHeapStatsCollector {
     size_t marked_bytes = 0;
     size_t compaction_freed_bytes = 0;
     size_t compaction_freed_pages = 0;
+    bool compaction_recorded_events = false;
     base::TimeDelta scope_data[kNumScopeIds];
     base::subtle::Atomic32 concurrent_scope_data[kNumConcurrentScopeIds]{0};
     BlinkGC::GCReason reason = static_cast<BlinkGC::GCReason>(0);

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

@@ -969,8 +969,10 @@ void UpdateHistograms(const ThreadHeapStatsCollector::Event& event) {
   DEFINE_STATIC_LOCAL(
       CustomCountHistogram, object_size_freed_by_heap_compaction,
       ("BlinkGC.ObjectSizeFreedByHeapCompaction", 1, 4 * 1024 * 1024, 50));
-  object_size_freed_by_heap_compaction.Count(
-      CappedSizeInKB(event.compaction_freed_bytes));
+  if (event.compaction_recorded_events) {
+    object_size_freed_by_heap_compaction.Count(
+        CappedSizeInKB(event.compaction_freed_bytes));
+  }
 
   DEFINE_STATIC_LOCAL(CustomCountHistogram, object_size_before_gc_histogram,
                       ("BlinkGC.ObjectSizeBeforeGC", 1, 4 * 1024 * 1024, 50));