Merge the tonic layer from skydart branch back to master

sky/engine/tonic/dart_gc_controller.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_controller.h"
+
+#include "dart/runtime/include/dart_api.h"
+#include "sky/engine/tonic/dart_gc_context.h"
+#include "sky/engine/tonic/dart_gc_visitor.h"
+#include "sky/engine/tonic/dart_wrappable.h"
+
+namespace blink {
+namespace {
+
+DartGCContext* g_gc_context = nullptr;
+
+DartWrappable* GetWrappable(intptr_t* fields) {
+  return reinterpret_cast<DartWrappable*>(fields[DartWrappable::kPeerIndex]);
+}
+
+void Visit(void* isolate_callback_data,
+           Dart_WeakPersistentHandle handle,
+           intptr_t native_field_count,
+           intptr_t* native_fields) {
+  if (!native_field_count)
+    return;
+  DCHECK(native_field_count == DartWrappable::kNumberOfNativeFields);
+  DartGCVisitor visitor(g_gc_context);
+  GetWrappable(native_fields)->AcceptDartGCVisitor(visitor);
+}
+
+}  // namespace
+
+void DartGCPrologue() {
+  Dart_EnterScope();
+  DCHECK(!g_gc_context);
+  g_gc_context = new DartGCContext();
+  Dart_VisitPrologueWeakHandles(Visit);
+}
+
+void DartGCEpilogue() {
+  delete g_gc_context;
+  g_gc_context = nullptr;
+  Dart_ExitScope();
+}
+
+}  // namespace blink