[heap] Add tracing scope to parallel scavenger.

Bug: chromium:758183
Change-Id: I5bfbff77562fdfd30587fdea475fb1b532e67b40
Reviewed-on: https://chromium-review.googlesource.com/809136
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49888}

src/heap/heap.cc

@@ -1909,7 +1909,9 @@ class ScavengingTask final : public ItemParallelJob::Task {
         barrier_(barrier) {}
 
   void RunInParallel() final {
-    // TODO(ulan): add GCTracer background scope.
+    GCTracer::BackgroundScope scope(
+        heap_->tracer(),
+        GCTracer::BackgroundScope::SCAVENGER_BACKGROUND_SCAVENGE_PARALLEL);
     double scavenging_time = 0.0;
     {
       barrier_->Start();