cc: Correct elastic overscroll synchronization.

cc/proto/layer_tree.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";
 
 package cc.proto;
 
 import "layer.proto";
 import "layer_selection_bound.proto";
 import "property_tree.proto";
 import "size.proto";
+import "vector2df.proto";
 
 option optimize_for = LITE_RUNTIME;
 
 message LayerTree {
   repeated int32 layers_that_should_push_properties = 1;
   optional bool in_paint_layer_contents = 2;
   optional int32 overscroll_elasticity_layer_id = 3;
   optional int32 page_scale_layer_id = 4;
   optional int32 inner_viewport_scroll_layer_id = 5;
   optional int32 outer_viewport_scroll_layer_id = 6;
@@ skipping 11 matching lines @@
   optional Size device_viewport_size = 18;
   optional uint32 background_color = 21; /* SkColor */
   optional bool has_transparent_background = 22;
   optional bool have_scroll_event_handlers = 23;
   optional uint32 touch_start_or_move_event_listener_properties = 24;
   optional uint32 wheel_event_listener_properties = 25;
   optional uint32 touch_end_or_cancel_event_listener_properties = 26;
   optional LayerNode root_layer = 27;
   optional LayerSelection selection = 29;
   optional PropertyTrees property_trees = 30;
+  optional Vector2dF elastic_overscroll = 31;
 }