Highlight individual work items in tracing of parallel GC.

This merely re-uses the same ScopeId as the task. Tracing shows these
as a nested layer with the same name. This is not the cleanest way to
do this but it makes the trace clearer for a minimal diff.

R=mlippautz@chromium.org

Bug: chromium:651354
Change-Id: Ib30ec7d04a30657a63a49aba9698cacd9af950d3
Reviewed-on: https://chromium-review.googlesource.com/904164
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51120}

src/heap/heap.cc

@@ -1981,6 +1981,9 @@ class ScavengingTask final : public ItemParallelJob::Task {
       TimedScope scope(&scavenging_time);
       PageScavengingItem* item = nullptr;
       while ((item = GetItem<PageScavengingItem>()) != nullptr) {
+        TRACE_BACKGROUND_GC(
+            heap_->tracer(),
+            GCTracer::BackgroundScope::SCAVENGER_BACKGROUND_SCAVENGE_PARALLEL);
         item->Process(scavenger_);
         item->MarkFinished();
       }

src/heap/mark-compact.cc

@@ -1818,6 +1818,9 @@ class YoungGenerationMarkingTask : public ItemParallelJob::Task {
       TimedScope scope(&marking_time);
       MarkingItem* item = nullptr;
       while ((item = GetItem<MarkingItem>()) != nullptr) {
+        TRACE_BACKGROUND_GC(
+            collector_->heap()->tracer(),
+            GCTracer::BackgroundScope::MINOR_MC_BACKGROUND_MARKING);
         item->Process(this);
         item->MarkFinished();
         EmptyLocalMarkingWorklist();
@@ -3204,6 +3207,7 @@ class PageEvacuationTask : public ItemParallelJob::Task {
     TRACE_BACKGROUND_GC(tracer_, evacuator_->GetBackgroundTracingScope());
     PageEvacuationItem* item = nullptr;
     while ((item = GetItem<PageEvacuationItem>()) != nullptr) {
+      TRACE_BACKGROUND_GC(tracer_, evacuator_->GetBackgroundTracingScope());
       evacuator_->EvacuatePage(item->page());
       item->MarkFinished();
     }
@@ -3520,6 +3524,7 @@ class PointersUpdatingTask : public ItemParallelJob::Task {
     TRACE_BACKGROUND_GC(tracer_, scope_);
     UpdatingItem* item = nullptr;
     while ((item = GetItem<UpdatingItem>()) != nullptr) {
+      TRACE_BACKGROUND_GC(tracer_, scope_);
       item->Process();
       item->MarkFinished();
     }