Distinguish weak references in heap snapshots, group GC roots.

src/profile-generator-inl.h

Index: src/profile-generator-inl.h
diff --git a/src/profile-generator-inl.h b/src/profile-generator-inl.h
index 88d6e879410911c83f795e5c062a9c9f1c48a117..a8c6c44096ba001709cf7bb37b7f9a994b12e8f7 100644
--- a/src/profile-generator-inl.h
+++ b/src/profile-generator-inl.h
@@ -95,6 +95,25 @@ CodeEntry* ProfileGenerator::EntryForVMState(StateTag tag) {
 }
 
 
+uint64_t HeapObjectsMap::GetNthGcSubrootId(int delta) {
+  return kGcRootsObjectId + delta * kObjectIdStep;
+}
+
+
+HeapObject* V8HeapExplorer::GetNthGcSubrootObject(int delta) {
+  return reinterpret_cast<HeapObject*>(
+      reinterpret_cast<char*>(kFirstGcSubrootObject) +
+      delta * HeapObjectsMap::kObjectIdStep);
+}
+
+
+int V8HeapExplorer::GetGcSubrootOrder(HeapObject* subroot) {
+  return (reinterpret_cast<char*>(subroot) -
+          reinterpret_cast<char*>(kFirstGcSubrootObject)) /
+      HeapObjectsMap::kObjectIdStep;
+}
+
+
 uint64_t HeapEntry::id() {
   union {
     Id stored_id;