Add support for (de)serializing LayerSelectionBound.

cc/proto/layer_selection_bound.proto

Index: cc/proto/layer_selection_bound.proto
diff --git a/cc/proto/rect.proto b/cc/proto/layer_selection_bound.proto
similarity index 50%
copy from cc/proto/rect.proto
copy to cc/proto/layer_selection_bound.proto
index a0194b185d8954112eef235f63a9c52ea1050b5b..397a1f82bef0758a29157456d173fd7e0a089650 100644
--- a/cc/proto/rect.proto
+++ b/cc/proto/layer_selection_bound.proto
@@ -5,13 +5,22 @@
 syntax = "proto2";
 
 import "point.proto";
-import "size.proto";
 
 option optimize_for = LITE_RUNTIME;
 
 package cc.proto;
 
-message Rect {
-  optional cc.proto.Point origin = 1;
-  optional cc.proto.Size size = 2;
+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;
 }