Introduce a flag for tracing retaining path in GC.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 1992c23925a061d7d334d12dc69b43dbc25b162e..7cf0e638c0fd84e9101db4631326a5fac707023e 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2094,10 +2094,16 @@ void MarkCompactCollector::ProcessEphemeralMarking(
   DCHECK(marking_deque_.IsEmpty() && !marking_deque_.overflowed());
   while (work_to_do) {
     if (!only_process_harmony_weak_collections) {
+#ifdef TRACE_RETAINING_PATH
+      heap()->SetCurrentRootRetainer(std::string("[weak global handles]"));
+#endif
       isolate()->global_handles()->IterateObjectGroups(
           visitor, &IsUnmarkedHeapObjectWithHeap);
       MarkImplicitRefGroups();
     }
+#ifdef TRACE_RETAINING_PATH
+    heap()->SetCurrentRootRetainer(std::string("[weak harmony collections]"));
+#endif
     ProcessWeakCollections();
     work_to_do = !marking_deque_.IsEmpty();
     ProcessMarkingDeque();
@@ -2106,6 +2112,9 @@ void MarkCompactCollector::ProcessEphemeralMarking(
 
 
 void MarkCompactCollector::ProcessTopOptimizedFrame(ObjectVisitor* visitor) {
+#ifdef TRACE_RETAINING_PATH
+  heap()->SetCurrentRootRetainer(std::string("[top optimized frame]"));
+#endif
   for (StackFrameIterator it(isolate(), isolate()->thread_local_top());
        !it.done(); it.Advance()) {
     if (it.frame()->type() == StackFrame::JAVA_SCRIPT) {
@@ -4412,5 +4421,12 @@ void SlotsBufferAllocator::DeallocateChain(SlotsBuffer** buffer_address) {
   }
   *buffer_address = NULL;
 }
+
+
+#ifdef TRACE_RETAINING_PATH
+void MarkingDeque::AddRetainedObject(HeapObject* obj) {
+  obj->GetHeap()->AddRetainedObject(obj);
+}
+#endif
 }
 }  // namespace v8::internal