[heap] Add tracing scope for concurrent marking.

Bug: chromium:758183
Change-Id: I00966a8831ff3d62c962573955e235538e4bf4bb
Reviewed-on: https://chromium-review.googlesource.com/809108Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49887}

src/heap/concurrent-marking.cc

@@ -428,7 +428,8 @@ ConcurrentMarking::ConcurrentMarking(Heap* heap, MarkingWorklist* shared,
 }
 
 void ConcurrentMarking::Run(int task_id, TaskState* task_state) {
-  // TODO(ulan): add GCTracer background scope.
+  GCTracer::BackgroundScope scope(
+      heap_->tracer(), GCTracer::BackgroundScope::MC_BACKGROUND_MARKING);
   size_t kBytesUntilInterruptCheck = 64 * KB;
   int kObjectsUntilInterrupCheck = 1000;
   LiveBytesMap* live_bytes = nullptr;