cc: Compute transform to target using effect id.

cc/trees/property_tree.cc

Index: cc/trees/property_tree.cc
diff --git a/cc/trees/property_tree.cc b/cc/trees/property_tree.cc
index ef0a2f32663e24fba6e2f4f5903ba248f0220806..a372541ed4391bfe5179c3cbc40826e1f18e06d9 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -773,9 +773,8 @@ void EffectTree::UpdateBackfaceVisibility(EffectNode* node,
             parent_transform_node->sorting_context_id ==
                 transform_node->sorting_context_id) {
           gfx::Transform surface_draw_transform;
-          transform_tree.ComputeTransform(
-              transform_node->id, transform_tree.TargetId(transform_node->id),
-              &surface_draw_transform);
+          property_trees()->ComputeTransformToTarget(
+              transform_node->id, node->target_id, &surface_draw_transform);
           node->hidden_by_backface_visibility =
               surface_draw_transform.IsBackFaceVisible();
         } else {
@@ -1876,4 +1875,24 @@ gfx::Transform PropertyTrees::ToScreenSpaceTransformWithoutSurfaceContentsScale(
   return screen_space_transform;
 }
 
+bool PropertyTrees::ComputeTransformToTarget(int transform_id,
+                                             int effect_id,
+                                             gfx::Transform* transform) const {
+  transform->MakeIdentity();
+  const EffectNode* effect_node = effect_tree.Node(effect_id);
+
+  DCHECK(effect_node->has_render_surface || effect_node->id == 0);
+
+  if (transform_id == effect_node->transform_id)
+    return true;
+
+  if (transform_id > effect_node->transform_id) {
+    return transform_tree.CombineTransformsBetween(
+        transform_id, effect_node->transform_id, transform);
+  }
+
+  return transform_tree.CombineInversesBetween(
+      transform_id, effect_node->transform_id, transform);

[ajuma, 2016/07/22 13:45:38]

Do we also need to handle the case where the two transform nodes don't have an
ancestor-descendant relationship? See
https://cs.chromium.org/chromium/src/cc/trees/property_tree.cc?rcl=0&l=260 for
an example.

[jaydasika, 2016/07/22 15:59:52]

This function calls CombineTransforms(/Inverses)Between which handles it.

+}
+
 }  // namespace cc