Improve perf of CalculateDrawPropertiesInternal when there are no scroll parents.

When we have scroll parents/children, we have to, potentially, reorder the children.
Previously, we had incurred a portion of this cost, even if we didn't have any scroll
parents/children in the tree. We would still populate a vector of pointers to the
children (representing the order that they would be visited in). Of course, this
order was the same as visiting layer->children() in order, so this work is wasted.
This patch stores a bool on a layer's draw properties indicating that it has a
child with a scroll parent. If this value is false, the reordering work may be
safely skipped.

BUG=305565

Review URL: https://codereview.chromium.org/26685002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227852 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 7 insertions, 0 deletions

diff --git a/cc/layers/draw_properties.h b/cc/layers/draw_properties.h
index bb8dcc0..b81bec7 100644
--- a/cc/layers/draw_properties.h
+++ b/cc/layers/draw_properties.h
@@ -30,6 +30,7 @@ struct CC_EXPORT DrawProperties {
         num_unclipped_descendants(0),
         can_draw_directly_to_backbuffer(false),
         layer_or_descendant_has_copy_request(false),
+        has_child_with_a_scroll_parent(false),
         sorted_for_recursion(false),
         index_of_first_descendants_addition(0),
         num_descendants_added(0),
@@ -103,6 +104,12 @@ struct CC_EXPORT DrawProperties {
   // present on it.
   bool layer_or_descendant_has_copy_request;
 
+  // This is true if the layer has any direct child that has a scroll parent.
+  // This layer will not be the scroll parent in this case. This information
+  // lets us avoid work in CalculateDrawPropertiesInternal -- if none of our
+  // children have scroll parents, we will not need to recur out of order.
+  bool has_child_with_a_scroll_parent;
+
   // This is true if the order (wrt to its siblings in the tree) in which the
   // layer will be visited while computing draw properties has been determined.
   bool sorted_for_recursion;