cc : Set 'needs update' on transform tree only when something changes

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 a572211abc73628963546b4121173a388ef50eb3..457b7611f082bcdf93f9c6861aaffcb7cf19dd44 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -679,15 +679,18 @@ void TransformTree::SetRootTransformsAndScales(
   gfx::Transform root_from_screen;
   bool invertible = root_to_screen.GetInverse(&root_from_screen);
   DCHECK(invertible);
-  SetToScreen(kRootNodeId, root_to_screen);
-  SetFromScreen(kRootNodeId, root_from_screen);
-  set_needs_update(true);
+  if (root_to_screen != ToScreen(kRootNodeId)) {
+    SetToScreen(kRootNodeId, root_to_screen);
+    SetFromScreen(kRootNodeId, root_from_screen);
+    set_needs_update(true);
+  }
 
   transform.ConcatTransform(root_from_screen);
   TransformNode* contents_root_node = Node(kContentsRootNodeId);
   if (contents_root_node->post_local != transform) {
     contents_root_node->post_local = transform;
     contents_root_node->needs_local_transform_update = true;
+    set_needs_update(true);
   }
 }