cc: Make LayerTreeImpl dump layer list to FrameViewer and Devtools

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 74603d51fbc0f1016e5cc45cfd7ffa8a6b09b4a4..35c5e7bc2256c34bb62f80d65a020d445cd3ad59 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -440,6 +440,14 @@ LayerListIterator<LayerImpl> LayerTreeImpl::end() {
   return LayerListIterator<LayerImpl>(nullptr);
 }
 
+const LayerListIterator<LayerImpl> LayerTreeImpl::begin() const {
+  return LayerListIterator<LayerImpl>(root_layer_);
+}
+
+const LayerListIterator<LayerImpl> LayerTreeImpl::end() const {
+  return LayerListIterator<LayerImpl>(nullptr);
+}
+
 LayerListReverseIterator<LayerImpl> LayerTreeImpl::rbegin() {
   return LayerListReverseIterator<LayerImpl>(root_layer_);
 }
@@ -1267,10 +1275,6 @@ void LayerTreeImpl::AsValueInto(base::trace_event::TracedValue* state) const {
   TracedValue::MakeDictIntoImplicitSnapshot(state, "cc::LayerTreeImpl", this);
   state->SetInteger("source_frame_number", source_frame_number_);
 
-  state->BeginDictionary("root_layer");
-  root_layer_->AsValueInto(state);
-  state->EndDictionary();
-
   state->BeginArray("render_surface_layer_list");
   LayerIterator end = LayerIterator::End(&render_surface_layer_list_);
   for (LayerIterator it = LayerIterator::Begin(&render_surface_layer_list_);
@@ -1290,6 +1294,14 @@ void LayerTreeImpl::AsValueInto(base::trace_event::TracedValue* state) const {
   for (const auto& swap_promise : pinned_swap_promise_list_)
     state->AppendDouble(swap_promise->TraceId());
   state->EndArray();
+
+  state->BeginArray("layers");
+  for (auto* layer : *this) {
+    state->BeginDictionary();
+    layer->AsValueInto(state);
+    state->EndDictionary();
+  }
+  state->EndArray();
 }
 
 bool LayerTreeImpl::DistributeRootScrollOffset(