Merge the tonic layer from skydart branch back to master

sky/engine/tonic/dart_gc_visitor.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "sky/engine/config.h"
+#include "sky/engine/tonic/dart_gc_visitor.h"
+
+#include "sky/engine/tonic/dart_gc_context.h"
+
+namespace blink {
+
+DartGCVisitor::DartGCVisitor(DartGCContext* context)
+    : context_(context), current_set_(nullptr) {
+}
+
+DartGCVisitor::~DartGCVisitor() {
+}
+
+void DartGCVisitor::AddToSetForRoot(const void* root,
+                                    Dart_WeakPersistentHandle handle) {
+  Dart_WeakReferenceSet set = context_->AddToSetForRoot(root, handle);
+  DCHECK(!current_set_ || current_set_ == set);
+  current_set_ = set;
+}
+
+}  // namespace blink