Merge the tonic layer from skydart branch back to master

sky/engine/tonic/dart_persistent_value.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_persistent_value.h"
+
+#include "sky/engine/tonic/dart_isolate_scope.h"
+#include "sky/engine/tonic/dart_state.h"
+
+namespace blink {
+
+DartPersistentValue::DartPersistentValue() : value_(nullptr) {
+}
+
+DartPersistentValue::DartPersistentValue(DartState* dart_state,
+                                         Dart_Handle value)
+    : value_(nullptr) {
+  Set(dart_state, value);
+}
+
+DartPersistentValue::~DartPersistentValue() {
+  Clear();
+}
+
+void DartPersistentValue::Set(DartState* dart_state, Dart_Handle value) {
+  DCHECK(is_empty());
+  dart_state_ = dart_state->GetWeakPtr();
+  value_ = Dart_NewPersistentHandle(value);
+}
+
+void DartPersistentValue::Clear() {
+  if (!value_ || !dart_state_.get())
+    return;
+
+  DartIsolateScope scope(dart_state_->isolate());
+  Dart_DeletePersistentHandle(value_);
+  dart_state_ = base::WeakPtr<DartState>();
+  value_ = nullptr;
+}
+
+Dart_Handle DartPersistentValue::Release() {
+  if (!value_)
+    return nullptr;
+  Dart_Handle local = Dart_HandleFromPersistent(value_);
+  Clear();
+  return local;
+}
+}