Use element id's for animations

cc/animation/element_id.h

Index: cc/animation/element_id.h
diff --git a/cc/animation/element_id.h b/cc/animation/element_id.h
new file mode 100644
index 0000000000000000000000000000000000000000..1da52a42e19da00f9c9dcf4f4b3b2cc30a573dfb
--- /dev/null
+++ b/cc/animation/element_id.h
@@ -0,0 +1,81 @@
+// 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_ANIMATION_ELEMENT_ID_H_
+#define CC_ANIMATION_ELEMENT_ID_H_
+
+#include <stddef.h>
+
+#include <cstdint>
+#include <functional>
+#include <iosfwd>
+#include <memory>
+
+#include "base/hash.h"
+#include "cc/base/cc_export.h"
+
+namespace base {
+class Value;
+namespace trace_event {
+class TracedValue;
+}  // namespace trace_event
+}  // namespace base
+
+namespace cc {
+
+namespace proto {
+class ElementId;
+}  // namespace proto
+
+// ------------------------------*IMPORTANT*---------------------------------
+// ElementId has a corresponding proto defined in cc/proto/element_id.proto.
+// When making any changes here, but sure to update the proto.
+
+// An "element" is really an animation target. It retains the name element to be
+// symmetric with ElementAnimations and blink::ElementAnimations, but is not
+// in fact tied to the notion of a blink element. It is also not associated with
+// the notion of a Layer. Ultimately, these ids will be used to look up the
+// property tree node associated with the given animation.
+//
+// These ids are chosen by cc's clients to permit the destruction and
+// restoration of cc entities (when visuals are hidden and shown) but maintain
+// stable identifiers. There will be a single layer for an ElementId, but
+// not every layer will have an id.
+struct CC_EXPORT ElementId {
+  ElementId(int primaryId, int secondaryId)
+      : primaryId(primaryId), secondaryId(secondaryId) {}
+  ElementId() : ElementId(0, 0) {}
+
+  bool operator==(const ElementId& o) const;
+  bool operator!=(const ElementId& o) const;
+  bool operator<(const ElementId& o) const;
+
+  // An ElementId's conversion to a boolean value depends only on its primaryId.
+  explicit operator bool() const;
+
+  void AddToTracedValue(base::trace_event::TracedValue* res) const;
+  std::unique_ptr<base::Value> AsValue() const;
+
+  void ToProtobuf(proto::ElementId* proto) const;
+  void FromProtobuf(const proto::ElementId& proto);
+
+  // The compositor treats this as an opaque handle and should not know how to
+  // interpret these bits. Non-blink cc clients typically operate in terms of
+  // layers and may set this value to match the client's layer id.
+  int primaryId;
+  int secondaryId;
+};
+
+CC_EXPORT ElementId LayerIdToElementIdForTesting(int layer_id);
+
+struct CC_EXPORT ElementIdHash {
+  size_t operator()(ElementId key) const;
+};
+
+// Stream operator so ElementId can be used in assertion statements.
+CC_EXPORT std::ostream& operator<<(std::ostream& out, const ElementId& id);
+
+}  // namespace cc
+
+#endif  // CC_ANIMATION_ELEMENT_ID_H_