Add SurfaceReference class.

cc/surfaces/surface_reference.h

Index: cc/surfaces/surface_reference.h
diff --git a/cc/surfaces/surface_reference.h b/cc/surfaces/surface_reference.h
new file mode 100644
index 0000000000000000000000000000000000000000..ac3bb773d10e80d9c3159bf36e1672ea44ed6438
--- /dev/null
+++ b/cc/surfaces/surface_reference.h
@@ -0,0 +1,65 @@
+// 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.
+
+#ifndef CC_SURFACES_SURFACE_REFERENCE_H_
+#define CC_SURFACES_SURFACE_REFERENCE_H_
+
+#include <string>
+
+#include "base/hash.h"
+#include "cc/surfaces/surface_id.h"
+#include "mojo/public/cpp/bindings/struct_traits.h"
+
+namespace cc {
+namespace mojom {
+class SurfaceReferenceDataView;
+}
+
+// Hold a reference from an embedding (parent) to embedded (child) surface.
+class SurfaceReference {
+ public:
+  SurfaceReference();
+  SurfaceReference(const SurfaceId& parent_id, const SurfaceId& child_id);
+  SurfaceReference(const SurfaceReference& other);
+
+  ~SurfaceReference();
+
+  const SurfaceId& parent_id() const { return parent_id_; }
+  const SurfaceId& child_id() const { return child_id_; }
+
+  size_t hash() const {
+    return base::HashInts(static_cast<uint64_t>(parent_id_.hash()),
+                          static_cast<uint64_t>(child_id_.hash()));
+  }
+
+  bool operator==(const SurfaceReference& other) const {
+    return parent_id_ == other.parent_id_ && child_id_ == other.child_id_;
+  }
+
+  bool operator!=(const SurfaceReference& other) const {
+    return !(*this == other);
+  }
+
+  bool operator<(const SurfaceReference& other) const {
+    return std::tie(parent_id_, child_id_) <
+           std::tie(other.parent_id_, other.child_id_);
+  }
+
+  std::string ToString() const;
+
+ private:
+  friend struct mojo::StructTraits<mojom::SurfaceReferenceDataView,
+                                   SurfaceReference>;
+
+  SurfaceId parent_id_;
+  SurfaceId child_id_;
+};
+
+struct SurfaceReferenceHash {
+  size_t operator()(const SurfaceReference& ref) const { return ref.hash(); }
+};
+
+}  // namespace cc
+
+#endif  // CC_SURFACES_SURFACE_REFERENCE_H_