cc : Rebuild property trees when opacity changes only if its required.

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index 4a52bf3a8669ca402f82b2951f055c9537baa63f..4f7c8efbfeb9fcc6b5ac6d492e4872e682a6c9a0 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -467,8 +467,22 @@ void Layer::SetOpacity(float opacity) {
   DCHECK(IsPropertyChangeAllowed());
   if (opacity_ == opacity)
     return;
+  bool force_rebuild = opacity == 1.f || opacity_ == 1.f;

[ajuma, 2016/05/16 22:05:20]

Please add a comment explaining why we need to unconditionally rebuild in this
case.

[jaydasika, 2016/05/17 01:40:00]

Done.

   opacity_ = opacity;
   SetSubtreePropertyChanged();

[ajuma, 2016/05/16 22:05:20]

I think this is only needed if we rebuild (Layer::SetTransform also calls this
even when not rebuilding, and that seems unnecessary too). Please move this call
and update the test to check for this.

[jaydasika, 2016/05/17 01:40:00]

I think we will need to mark both on the layer and property trees because if we
rebuild property trees for some other reason, we should not loose the damage
tracking info.

+  if (layer_tree_host_ && !force_rebuild) {
+    PropertyTrees* property_trees = layer_tree_host_->property_trees();
+    auto effect_id_to_index = property_trees->effect_id_to_index_map.find(id());
+    if (effect_id_to_index != property_trees->effect_id_to_index_map.end()) {
+      EffectNode* node =
+          property_trees->effect_tree.Node(effect_id_to_index->second);
+      node->data.opacity = opacity;
+      node->data.effect_changed = true;
+      property_trees->effect_tree.set_needs_update(true);
+      SetNeedsCommitNoRebuild();
+      return;
+    }
+  }
   SetNeedsCommit();
 }