Keeping track of descendants that draw content instead of recalcualting

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index bd9a2d7b21e220c3ff93f4e866d2599d8b709a6f..e6e60f5f3ef7cc1e474c2b5c4a2aef192906b3b7 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -64,6 +64,7 @@ LayerImpl::LayerImpl(LayerTreeImpl* tree_impl, int id)
       background_color_(0),
       opacity_(1.0),
       blend_mode_(SkXfermode::kSrcOver_Mode),
+      num_descendants_that_draw_content_(0),
       draw_depth_(0.f),
       needs_push_properties_(false),
       num_dependents_need_push_properties_(0),
@@ -176,6 +177,13 @@ void LayerImpl::SetScrollChildren(std::set<LayerImpl*>* children) {
   SetNeedsPushProperties();
 }
 
+void LayerImpl::SetNumDescendantsThatDrawContent(int num_descendants) {
+  if (num_descendants_that_draw_content_ == num_descendants)
+    return;
+  num_descendants_that_draw_content_ = num_descendants;
+  SetNeedsPushProperties();
+}
+
 void LayerImpl::SetClipParent(LayerImpl* ancestor) {
   if (clip_parent_ == ancestor)
     return;
@@ -532,6 +540,7 @@ void LayerImpl::PushPropertiesTo(LayerImpl* layer) {
       scroll_offset_, layer->ScrollDelta() - layer->sent_scroll_delta());
   layer->SetSentScrollDelta(gfx::Vector2d());
   layer->Set3dSortingContextId(sorting_context_id_);
+  layer->SetNumDescendantsThatDrawContent(num_descendants_that_draw_content_);
 
   LayerImpl* scroll_parent = NULL;
   if (scroll_parent_) {
@@ -1521,6 +1530,10 @@ void LayerImpl::RunMicroBenchmark(MicroBenchmarkImpl* benchmark) {
   benchmark->RunOnLayer(this);
 }
 
+int LayerImpl::NumDescendantsThatDrawContent() const {
+  return num_descendants_that_draw_content_;
+}
+
 void LayerImpl::NotifyAnimationFinished(
     base::TimeTicks monotonic_time,
     Animation::TargetProperty target_property) {