Disable overlay scrollbars in Blink when hidden by the compositor.

cc/proto/begin_main_frame_and_commit_state.proto

 // 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.
 
 syntax = "proto2";
 
 import "vector2d.proto";
 import "vector2df.proto";
 
 option optimize_for = LITE_RUNTIME;
 
 package cc.proto;
 
 message ScrollUpdateInfo {
   optional int64 layer_id = 1;
   optional Vector2d scroll_delta = 2;
 }
 
+message ScrollbarsUpdateInfo {
+  optional int64 layer_id = 1;
+  optional bool hidden = 2;
+}
+
 message ScrollAndScaleSet {
   repeated ScrollUpdateInfo scrolls = 1;
   optional float page_scale_delta = 2;
   optional Vector2dF elastic_overscroll_delta = 3;
   optional float top_controls_delta = 4;
+  repeated ScrollbarsUpdateInfo scrollbars = 5;
 
   // TODO(khushalsagar): Do we need to send swap promises?
   // See crbug/576999.
 }
 
 message BeginFrameArgs {
   enum BeginFrameArgsType {
     INVALID = 1;
     NORMAL = 2;
     MISSED = 3;
     BEGIN_FRAME_ARGS_TYPE_MAX = 100;
   }
 
   optional int64 frame_time = 1;
   optional int64 deadline = 2;
   optional int64 interval = 3;
   optional BeginFrameArgsType type = 4;
   optional bool on_critical_path = 5;
 }
 
 message BeginMainFrameAndCommitState {
   optional int64 begin_frame_id = 1;
   optional BeginFrameArgs begin_frame_args = 2;
   optional ScrollAndScaleSet scroll_info = 3;
   optional int64 memory_allocation_limit_bytes = 4;
   optional bool evicted_ui_resources = 5;
 }