blink: remove unnecessary root-layer checks

The root layer is always a scroll container, so checking for
root-layer in addition to scroll container is unnecessary.

BUG=1087667

Change-Id: I179f9f5aaa9e65481e753067db46c98a756d8a67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432412
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811826}

third_party/blink/renderer/core/paint/compositing/compositing_inputs_updater.cc

@@ -95,7 +95,7 @@ void CompositingInputsUpdater::ApplyAncestorInfoToSelfAndAncestorsRecursively(
     geometry_map_->PushMappingsToAncestor(layer, layer->Parent());
   UpdateAncestorInfo(layer, update_type, info);
   if (layer != compositing_inputs_root_ &&
-      (layer->IsRootLayer() || layer->GetLayoutObject().IsScrollContainer()))
+      layer->GetLayoutObject().IsScrollContainer())
     info.last_scroll_container_layer = layer;
 }
 
@@ -151,7 +151,7 @@ void CompositingInputsUpdater::UpdateSelfAndDescendantsRecursively(
       geometry_map_->PushMappingsToAncestor(layer, layer->Parent());
     UpdateAncestorInfo(layer, update_type, info);
   }
-  if (layer->IsRootLayer() || layout_object.IsScrollContainer())
+  if (layout_object.IsScrollContainer())
     info.last_scroll_container_layer = layer;
 
   PaintLayerCompositor* compositor =

third_party/blink/renderer/core/paint/pre_paint_tree_walk.cc

@@ -386,7 +386,7 @@ void PrePaintTreeWalk::UpdateAuxiliaryObjectProperties(
     // update layer position and ancestor inputs updates in the same walk).
     paint_layer->UpdateLayerPosition();
   }
-  if (paint_layer->IsRootLayer() || object.IsScrollContainer())
+  if (object.IsScrollContainer())
     context.ancestor_scroll_container_paint_layer = paint_layer;
 }