Add/remove surface references via MojoCompositorFrameSink.

cc/surfaces/surface_reference.cc

+// Copyright 2016 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 "cc/surfaces/surface_reference.h"
+
+namespace cc {
+
+SurfaceReference::SurfaceReference() {}
+
+SurfaceReference::SurfaceReference(const SurfaceId& parent_id,
+                                   const SurfaceId& child_id)
+    : parent_id_(parent_id), child_id_(child_id) {}
+
+SurfaceReference::SurfaceReference(const SurfaceReference& other) = default;
+
+SurfaceReference::~SurfaceReference() {}
+
+std::string SurfaceReference::ToString() const {
+  return parent_id_.ToString() + " to " + child_id_.ToString();
+}
+
+}  // namespace cc