Reland of Use Layer::INVALID_ID instead of 0 for invalid layers in WebScrollbarLayerImpl

cc/blink/web_scrollbar_layer_impl.cc

 // Copyright 2014 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/blink/web_scrollbar_layer_impl.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "cc/blink/scrollbar_impl.h"
@@ skipping 21 matching lines @@
 namespace cc_blink {
 
 WebScrollbarLayerImpl::WebScrollbarLayerImpl(
     std::unique_ptr<blink::WebScrollbar> scrollbar,
     blink::WebScrollbarThemePainter painter,
     std::unique_ptr<blink::WebScrollbarThemeGeometry> geometry)
     : layer_(new WebLayerImpl(PaintedScrollbarLayer::Create(
           base::MakeUnique<ScrollbarImpl>(std::move(scrollbar),
                                           painter,
                                           std::move(geometry)),
-          0))) {}
+          cc::Layer::INVALID_ID))) {}
 
 WebScrollbarLayerImpl::WebScrollbarLayerImpl(
     std::unique_ptr<blink::WebScrollbar> scrollbar,
     blink::WebScrollbarThemePainter painter,
     std::unique_ptr<blink::WebScrollbarThemeGeometry> geometry,
     bool)
     : layer_(new WebLayerImpl(PaintedOverlayScrollbarLayer::Create(
           base::MakeUnique<ScrollbarImpl>(std::move(scrollbar),
                                           painter,
                                           std::move(geometry)),
-          0))) {}
+          cc::Layer::INVALID_ID))) {}
 
 WebScrollbarLayerImpl::WebScrollbarLayerImpl(
     blink::WebScrollbar::Orientation orientation,
     int thumb_thickness,
     int track_start,
     bool is_left_side_vertical_scrollbar)
     : layer_(new WebLayerImpl(
           SolidColorScrollbarLayer::Create(ConvertOrientation(orientation),
                                            thumb_thickness,
                                            track_start,
                                            is_left_side_vertical_scrollbar,
-                                           0))) {}
+                                           cc::Layer::INVALID_ID))) {}
 
 WebScrollbarLayerImpl::~WebScrollbarLayerImpl() {
 }
 
 blink::WebLayer* WebScrollbarLayerImpl::layer() {
   return layer_.get();
 }
 
 void WebScrollbarLayerImpl::setScrollLayer(blink::WebLayer* layer) {
   cc::Layer* scroll_layer =
       layer ? static_cast<WebLayerImpl*>(layer)->layer() : 0;
   layer_->layer()->ToScrollbarLayer()->SetScrollLayer(
       scroll_layer ? scroll_layer->id() : cc::Layer::INVALID_ID);
 }
 
 }  // namespace cc_blink