Add support for (de)serializing LayerSelection.

cc/proto/layer_selection_bound.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 "point.proto";
 
 option optimize_for = LITE_RUNTIME;
 
 package cc.proto;
 
 enum SelectionBoundType {
   UNKNOWN = 0;
   LEFT = 1;
   RIGHT = 2;
   CENTER = 3;
   EMPTY = 4;
 }
 
 message LayerSelectionBound {
   optional SelectionBoundType type = 1;
   optional Point edge_top = 2;
   optional Point edge_bottom = 3;
   optional int32 layer_id = 4;
 }
+
+message LayerSelection {
+  optional LayerSelectionBound start = 1;
+  optional LayerSelectionBound end = 2;
+  optional bool is_editable = 3;
+  optional bool is_empty_text_form_control = 4;
+}