Add support for (de)serializing HeadsUpDisplayLayer

cc/proto/layer.proto

 // 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.
 
 syntax = "proto2";
 
 import "display_list_recording_source.proto";
 import "layer_position_constraint.proto";
 import "point3f.proto";
 import "pointf.proto";
 import "region.proto";
 import "rect.proto";
 import "scroll_offset.proto";
 import "size.proto";
 import "skxfermode.proto";
 import "transform.proto";
 import "vector2df.proto";
 
 package cc.proto;
 
 option optimize_for = LITE_RUNTIME;
 
 // Identifies the type of cc:Layer a LayerNode represents. It is used to
 // facilitate reconstruction of a Layer of the correct type on the client.
 enum LayerType {
   UNKNOWN = 0;
   LAYER = 1;
   PICTURE_LAYER = 2;
+  HEADS_UP_DISPLAY_LAYER = 3;
 
   // TODO(nyquist): Add the rest of the necessary LayerTypes.
 };
 
 // Hierarchical structure for serializing the Layer tree.
 message LayerNode {
   // required
   optional int32 id = 1;
   // required
   optional LayerType type = 2;
@@ skipping 92 matching lines @@
 }
 
 message PictureLayerProperties {
   optional DisplayListRecordingSource recording_source = 1;
   optional Region invalidation = 2;
   optional Rect last_updated_visible_layer_rect = 3;
   optional bool is_mask = 4;
   optional bool nearest_neighbor = 5;
 
   optional int64 update_source_frame_number = 6;
-}
+}