[heap] Add tracing scope to minor MC parallel marking.

Bug: chromium:758183
Change-Id: I7e0d5d6cfbef82895f76bd020a8ad22e4be013e2
Reviewed-on: https://chromium-review.googlesource.com/809151Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49897}

src/heap/mark-compact.cc

@@ -1865,7 +1865,9 @@ class YoungGenerationMarkingTask : public ItemParallelJob::Task {
   }
 
   void RunInParallel() override {
-    // TODO(ulan): add GCTracer background scope.
+    GCTracer::BackgroundScope scope(
+        collector_->heap()->tracer(),
+        GCTracer::BackgroundScope::MINOR_MC_BACKGROUND_MARKING);
     double marking_time = 0.0;
     {
       TimedScope scope(&marking_time);