Call setNeedsRedraw when scrollbar opacity changes.

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 2304 matching lines @@
 }
 
 std::vector<base::Closure>
 LayerTreeHostImpl::TakeCompletedImageDecodeCallbacks() {
   auto result = std::move(completed_image_decode_callbacks_);
   completed_image_decode_callbacks_.clear();
   return result;
 }
 
 void LayerTreeHostImpl::DidChangeScrollbarVisibility() {
+  // Need a commit since input handling for scrollbars is handled in Blink so
+  // we need to communicate to Blink when the compositor shows/hides the
+  // scrollbars.
   client_->SetNeedsCommitOnImplThread();
 }
 
 void LayerTreeHostImpl::CleanUpTileManagerAndUIResources() {
   ClearUIResources();
   tile_manager_.FinishTasksAndCleanUp();
   resource_pool_ = nullptr;
   single_thread_synchronous_task_graph_runner_ = nullptr;
   image_decode_cache_ = nullptr;
 
@@ skipping 1877 matching lines @@
       worker_context_visibility_ =
           worker_context->CacheController()->ClientBecameVisible();
     } else {
       worker_context->CacheController()->ClientBecameNotVisible(
           std::move(worker_context_visibility_));
     }
   }
 }
 
 }  // namespace cc