[SPv1] Always set a CompositorElementId on main graphics layers; use PaintLayer id.

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 338 matching lines @@
   ScrollbarTheme& theme = scrollbar.GetTheme();
   WebScrollbarThemePainter painter(theme, scrollbar, device_scale_factor);
   std::unique_ptr<WebScrollbarThemeGeometry> geometry(
       WebScrollbarThemeGeometryNative::Create(theme));
 
   std::unique_ptr<WebScrollbarLayer> scrollbar_layer;
   if (theme.UsesOverlayScrollbars() && theme.UsesNinePatchThumbResource()) {
     scrollbar_layer =
         Platform::Current()->CompositorSupport()->CreateOverlayScrollbarLayer(
             WebScrollbarImpl::Create(&scrollbar), painter, std::move(geometry));
-    scrollbar_layer->SetElementId(CompositorElementIdFromDOMNodeId(
+    scrollbar_layer->SetElementId(CompositorElementIdFromScrollbarId(
         NextScrollbarId(), CompositorElementIdNamespace::kScrollbar));
   } else {
     scrollbar_layer =
         Platform::Current()->CompositorSupport()->CreateScrollbarLayer(
             WebScrollbarImpl::Create(&scrollbar), painter, std::move(geometry));
-    scrollbar_layer->SetElementId(CompositorElementIdFromDOMNodeId(
+    scrollbar_layer->SetElementId(CompositorElementIdFromScrollbarId(
         NextScrollbarId(), CompositorElementIdNamespace::kScrollbar));
   }
   GraphicsLayer::RegisterContentsLayer(scrollbar_layer->Layer());
   return scrollbar_layer;
 }
 
 std::unique_ptr<WebScrollbarLayer>
 ScrollingCoordinator::CreateSolidColorScrollbarLayer(
     ScrollbarOrientation orientation,
     int thumb_thickness,
     int track_start,
     bool is_left_side_vertical_scrollbar) {
   WebScrollbar::Orientation web_orientation =
       (orientation == kHorizontalScrollbar) ? WebScrollbar::kHorizontal
                                             : WebScrollbar::kVertical;
   std::unique_ptr<WebScrollbarLayer> scrollbar_layer =
       Platform::Current()->CompositorSupport()->CreateSolidColorScrollbarLayer(
           web_orientation, thumb_thickness, track_start,
           is_left_side_vertical_scrollbar);
-  scrollbar_layer->SetElementId(CompositorElementIdFromDOMNodeId(
+  scrollbar_layer->SetElementId(CompositorElementIdFromScrollbarId(
       NextScrollbarId(), CompositorElementIdNamespace::kScrollbar));
   GraphicsLayer::RegisterContentsLayer(scrollbar_layer->Layer());
   return scrollbar_layer;
 }
 
 static void DetachScrollbarLayer(GraphicsLayer* scrollbar_graphics_layer) {
   DCHECK(scrollbar_graphics_layer);
 
   scrollbar_graphics_layer->SetContentsToPlatformLayer(nullptr);
   scrollbar_graphics_layer->SetDrawsContent(true);
@@ skipping 818 matching lines @@
                                       ->LayerForScrolling())
                      : nullptr) {
     return WebSize(
                frame_view->LayoutViewportScrollableArea()->ContentsSize()) !=
            scroll_layer->Bounds();
   }
   return false;
 }
 
 }  // namespace blink