Attempt to simplify source_index calculation

cc/trees/property_tree_builder.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/trees/property_tree_builder.h"
 
 #include <stddef.h>
 
 #include <map>
 #include <set>
@@ skipping 519 matching lines @@
       IsAtBoundaryOf3dRenderingContext(layer);
 
   DCHECK(!is_scrollable || is_snapped);
   bool requires_node = is_root || is_snapped || has_significant_transform ||
                        has_any_transform_animation || has_surface || is_fixed ||
                        is_page_scale_layer || is_overscroll_elasticity_layer ||
                        has_proxied_transform_related_property ||
                        scroll_child_has_different_target || is_sticky ||
                        is_at_boundary_of_3d_rendering_context;
 
+  int parent_index = TransformTree::kRootNodeId;
+  int source_index = TransformTree::kRootNodeId;
+  gfx::Vector2dF source_offset;
+
   LayerType* transform_parent = GetTransformParent(data_from_ancestor, layer);
-  DCHECK(is_root || transform_parent);
+  DCHECK_EQ(is_root, !transform_parent);
 
-  int parent_index = TransformTree::kRootNodeId;
-  if (transform_parent)
+  if (transform_parent) {
     parent_index = transform_parent->transform_tree_index();
-
-  int source_index = parent_index;
-
-  gfx::Vector2dF source_offset;
-  if (transform_parent) {
-    if (ScrollParent(layer)) {
-      LayerType* source = Parent(layer);
-      source_offset += source->offset_to_transform_parent();
-      source_index = source->transform_tree_index();
-    } else if (!is_fixed) {
-      source_offset = transform_parent->offset_to_transform_parent();
-    } else {
-      source_offset = data_from_ancestor.transform_tree_parent
-                          ->offset_to_transform_parent();
-      source_index =
-          data_from_ancestor.transform_tree_parent->transform_tree_index();
-    }
+    // Because Blink still provides positions with respect to the parent layer,
+    // we track both a parent TransformNode (which is the parent in the
+    // TransformTree) and a 'source' TransformNode (which is the TransformNode
+    // for the parent in the Layer tree).
+    source_index = Parent(layer)->transform_tree_index();
+    source_offset = Parent(layer)->offset_to_transform_parent();
   }
 
   if (IsContainerForFixedPositionLayers(layer) || is_root) {
     data_for_children->affected_by_inner_viewport_bounds_delta =
         layer == data_from_ancestor.inner_viewport_scroll_layer;
     data_for_children->affected_by_outer_viewport_bounds_delta =
         layer == data_from_ancestor.outer_viewport_scroll_layer;
     if (is_scrollable) {
       DCHECK(!is_root);
       DCHECK(Transform(layer).IsIdentity());
@@ skipping 959 matching lines @@
       root_layer, page_scale_layer, inner_viewport_scroll_layer,
       outer_viewport_scroll_layer, overscroll_elasticity_layer,
       elastic_overscroll, page_scale_factor, device_scale_factor, viewport,
       device_transform, property_trees, color);
   property_trees->effect_tree.CreateOrReuseRenderSurfaces(
       &render_surfaces, root_layer->layer_tree_impl());
   property_trees->ResetCachedData();
 }
 
 }  // namespace cc