Add ThreadBarrier; use in GCMarker and unit test

runtime/vm/gc_marker.h

Index: runtime/vm/gc_marker.h
diff --git a/runtime/vm/gc_marker.h b/runtime/vm/gc_marker.h
index 87fed84feff0c327c3b5d7af13ef1186db014825..fb0b85d4c3989a7c6ac0c5b260893b43acbe9794 100644
--- a/runtime/vm/gc_marker.h
+++ b/runtime/vm/gc_marker.h
@@ -23,7 +23,7 @@ class RawWeakProperty;
 class GCMarker : public ValueObject {
  public:
   explicit GCMarker(Heap* heap)
-      : heap_(heap), marked_bytes_(0), done_count_(0) { }
+      : heap_(heap), marked_bytes_(0) { }
   ~GCMarker() { }
 
   void MarkObjects(Isolate* isolate,
@@ -46,20 +46,12 @@ class GCMarker : public ValueObject {
   void ProcessWeakTables(PageSpace* page_space);
   void ProcessObjectIdTable(Isolate* isolate);
 
-  // Synchronization between GCMarker's main thread and its marking tasks.
-  // Called by main thread: wait for 'num_tasks' tasks, then let them resume.
-  void MainSync(intptr_t num_tasks);
-  // Called by tasks: notify main thread; wait until it lets us continue.
-  void TaskSync();
-  // Called by tasks: notify main thread, but don't wait (used for exiting).
-  void TaskNotifyDone();
   // Called by anyone: finalize and accumulate stats from 'visitor'.
   void FinalizeResultsFrom(MarkingVisitor* visitor);
 
   Monitor monitor_;  // Protects marked_bytes_ and done_count_.
   Heap* heap_;
   uintptr_t marked_bytes_;
-  intptr_t done_count_;
 
   friend class MarkTask;
   DISALLOW_IMPLICIT_CONSTRUCTORS(GCMarker);