[cc] Change semantics of fixed-pos container layer

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 378 matching lines @@
     // 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());
-      data_for_children->transform_fixed_parent = Parent(layer);
-    } else {
-      data_for_children->transform_fixed_parent = layer;
-    }
+    data_for_children->transform_fixed_parent = layer;
   }
   data_for_children->transform_tree_parent = layer;
 
   if (!requires_node) {
     data_for_children->should_flatten |= ShouldFlattenTransform(layer);
     gfx::Vector2dF local_offset = layer->position().OffsetFromOrigin() +
                                   Transform(layer).To2dTranslation();
     gfx::Vector2dF source_to_parent;
     if (source_index != parent_index) {
       gfx::Transform to_parent;
@@ skipping 965 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