cc: Make impl-thread property trees handle fixed-position layers

cc/trees/property_tree_builder.cc

Index: cc/trees/property_tree_builder.cc
diff --git a/cc/trees/property_tree_builder.cc b/cc/trees/property_tree_builder.cc
index 7f48b64b07bd0b959f0cef2b7a2f09b898239319..70dd418375148efb4a9d9be45fb9328146029411 100644
--- a/cc/trees/property_tree_builder.cc
+++ b/cc/trees/property_tree_builder.cc
@@ -212,8 +212,13 @@ bool AddTransformNodeIfNeeded(
   }
 
   if (layer->IsContainerForFixedPositionLayers() || is_root) {
-    DCHECK(!is_scrollable || layer->transform().IsIdentity());
-    data_for_children->transform_fixed_parent = layer;
+    if (is_scrollable) {
+      DCHECK(!is_root);
+      DCHECK(layer->transform().IsIdentity());
+      data_for_children->transform_fixed_parent = layer->parent();
+    } else {
+      data_for_children->transform_fixed_parent = layer;
+    }
   }
   data_for_children->transform_tree_parent = layer;
 
@@ -386,6 +391,9 @@ void BuildPropertyTreesTopLevelInternal(LayerType* root_layer,
                                         const gfx::Rect& viewport,
                                         const gfx::Transform& device_transform,
                                         PropertyTrees* property_trees) {
+  if (!property_trees->needs_rebuild)
+    return;

[ajuma, 2015/05/29 17:47:08]

This change causes about 16 unit tests to fail, since they rely on property
trees being rebuilt every frame. I'm not sure what the best approach is here.
One option would be to change those tests so that they explicitly set
needs_rebuild. Alternatively, we could add a way for tests that don't want to
rebuild (like LayerPositionConstraintTests) to opt out. Making the default be
"don't rebuild" seems better in the sense that non-test impl property trees
aren't rebuilt, so tests that want to rebuild anyway should be making a
conscious choice.

[enne (OOO), 2015/05/29 20:07:53]

Agreed that the default should be don't rebuild unless the flag is set.  Tests
that are doing weird things should mark rebuild explicitly if they really need
it.

+
   property_trees->sequence_number++;
 
   DataForRecursion<LayerType> data_for_recursion;
@@ -433,10 +441,6 @@ void PropertyTreeBuilder::BuildPropertyTrees(
     const gfx::Rect& viewport,
     const gfx::Transform& device_transform,
     PropertyTrees* property_trees) {
-  // TODO(enne): hoist this out of here
-  if (!property_trees->needs_rebuild)
-    return;
-
   BuildPropertyTreesTopLevelInternal(
       root_layer, page_scale_layer, page_scale_factor, device_scale_factor,
       viewport, device_transform, property_trees);