cc: DCHECK that the OutputSurface was released in ~LayerTreeHostImpl().

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 c39359ae73e2cd33bd8fcf55b94150afc51c9069..3d0b9d9e3a7c976c977b72962178817f5e128f91 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -100,6 +100,18 @@ void LayerTreeImpl::Shutdown() {
 }
 
 void LayerTreeImpl::ReleaseResources() {
+#if DCHECK_IS_ON()
+  // These DCHECKs catch tests that add layers to the tree but fail to build the

[enne (OOO), 2016/06/27 22:22:20]

Nice!

+  // layer list afterward.
+  LayerListIterator<LayerImpl> it(root_layer_);
+  size_t i = 0;
+  for (; it != LayerListIterator<LayerImpl>(nullptr); ++it, ++i) {
+    DCHECK_LT(i, layer_list_.size());
+    DCHECK_EQ(layer_list_[i], *it);
+  }
+#endif
+
+  DCHECK_EQ(!root_layer_, LayerListIsEmpty());
   if (!LayerListIsEmpty()) {
     LayerTreeHostCommon::CallFunctionForEveryLayer(
         this, [](LayerImpl* layer) { layer->ReleaseResources(); });