Collect samples for background threads.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index c9ac9faea5d67e5d3fcc59677bf83b27d13f4848..e21d0a26dc464cbd7fb7b70f77a66dc096a7facc 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -19,6 +19,7 @@
 #include "vm/thread_barrier.h"
 #include "vm/thread_pool.h"
 #include "vm/thread_registry.h"
+#include "vm/timeline.h"
 #include "vm/visitor.h"
 #include "vm/object_id_ring.h"
 
@@ -602,7 +603,9 @@ class MarkTask : public ThreadPool::Task {
     bool result = Thread::EnterIsolateAsHelper(isolate_, true);
     ASSERT(result);
     {
-      StackZone stack_zone(Thread::Current());
+      Thread* thread = Thread::Current();
+      TimelineDurationScope tds(thread, Timeline::GetGCStream(), "MarkTask");
+      StackZone stack_zone(thread);
       Zone* zone = stack_zone.GetZone();
       SkippedCodeFunctions* skipped_code_functions =
           collect_code_ ? new(zone) SkippedCodeFunctions() : NULL;