cc : Use screen space transform for computing translation

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 12e42508beb4fbf3146215d675ecbebf7ff48e3f..f23bc969cedee91ff6adee4ad704770b6f5190f4 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -161,10 +161,25 @@ bool TransformTree::ComputeTransform(int source_id,
 bool TransformTree::ComputeTranslation(int source_id,
                                        int dest_id,
                                        gfx::Transform* transform) const {
-  bool success = ComputeTransform(source_id, dest_id, transform);
+  // Since the transform between source and destination is at most translation,
+  // flattenning will not affect the result and we can use the screen space
+  // transforms.
+  transform->MakeIdentity();
+  if (source_id == dest_id)
+    return true;
+
+  const TransformNode* dest = Node(dest_id);
+  if (dest->ancestors_are_invertible) {
+    if (source_id != kInvalidNodeId)
+      transform->ConcatTransform(ToScreen(source_id));
+    if (dest_id != kInvalidNodeId)
+      transform->ConcatTransform(FromScreen(dest_id));
+  } else {
+    return false;
+  }
   DCHECK(
       transform->IsApproximatelyIdentityOrTranslation(SkDoubleToMScalar(1e-4)));
-  return success;
+  return true;
 }
 
 bool TransformTree::NeedsSourceToParentUpdate(TransformNode* node) {