Revert "Make all references from root shortcuts (except one to GC roots)"

This reverts commit 940418d2e1150499a28cca60b5dd138d351ec652. It resulted in orphan (Document DOM trees) and (Detached DOM trees) nodes.

BUG=chromium:414190
LOG=N
TBR=loislo@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/639753007

Cr-Commit-Position: refs/heads/master@{#24903}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24903 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap-snapshot-generator.cc

@@ -2517,8 +2517,10 @@ void NativeObjectsExplorer::SetRootNativeRootsReference() {
     HeapEntry* group_entry =
         filler_->FindOrAddEntry(group_info, native_entries_allocator_);
     DCHECK(group_entry != NULL);
-    filler_->SetNamedAutoIndexReference(
-        HeapGraphEdge::kShortcut, snapshot_->root()->index(), group_entry);
+    filler_->SetIndexedAutoIndexReference(
+        HeapGraphEdge::kElement,
+        snapshot_->root()->index(),
+        group_entry);
   }
 }