Fix cc scrollbar layer issues with initialization, and use element ids throughout.

cc/layers/painted_overlay_scrollbar_layer.cc

 // 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.
 
 #include "cc/layers/painted_overlay_scrollbar_layer.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "cc/base/math_util.h"
@@ skipping 47 matching lines @@
     return;
 
   scroll_element_id_ = element_id;
   SetNeedsFullTreeSync();
 }
 
 bool PaintedOverlayScrollbarLayer::OpacityCanAnimateOnImplThread() const {
   return scrollbar_->IsOverlay();
 }
 
-bool PaintedOverlayScrollbarLayer::AlwaysUseActiveTreeOpacity() const {
-  return true;
-}
-
 ScrollbarOrientation PaintedOverlayScrollbarLayer::orientation() const {
   return scrollbar_->Orientation();
 }
 
 void PaintedOverlayScrollbarLayer::PushPropertiesTo(LayerImpl* layer) {
   Layer::PushPropertiesTo(layer);
 
   PaintedOverlayScrollbarLayerImpl* scrollbar_layer =
       static_cast<PaintedOverlayScrollbarLayerImpl*>(layer);
 
@@ skipping 83 matching lines @@
 
   thumb_resource_ = ScopedUIResource::Create(
       layer_tree_host()->GetUIResourceManager(), UIResourceBitmap(skbitmap));
 
   SetNeedsPushProperties();
 
   return true;
 }
 
 }  // namespace cc