Merge the tonic layer from skydart branch back to master

sky/engine/tonic/dart_gc_context.h

+// 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.
+
+#ifndef SKY_ENGINE_TONIC_DART_GC_CONTEXT_H_
+#define SKY_ENGINE_TONIC_DART_GC_CONTEXT_H_
+
+#include "base/macros.h"
+#include "dart/runtime/include/dart_api.h"
+#include "sky/engine/wtf/HashMap.h"
+
+namespace blink {
+
+class DartGCContext {
+ public:
+  DartGCContext();
+  ~DartGCContext();
+
+  Dart_WeakReferenceSet AddToSetForRoot(const void* root,
+                                        Dart_WeakPersistentHandle handle);
+
+ private:
+  Dart_WeakReferenceSetBuilder builder_;
+  HashMap<const void*, Dart_WeakReferenceSet> references_;
+
+  DISALLOW_COPY_AND_ASSIGN(DartGCContext);
+};
+
+}  // namespace blink
+
+#endif  // SKY_ENGINE_TONIC_DART_GC_CONTEXT_H_