cc: Remove some DCHECKs in CopyOutputRequest code.

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 9da16b4deddc42ca38b89da28d19a2835f01dbb5..d69e7ff2e863c4410de1242e36c74666c2a2af52 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -565,9 +565,10 @@ void LayerTreeImpl::AddLayerWithCopyOutputRequest(LayerImpl* layer) {
   // they are aborted if not serviced during draw.
   DCHECK(IsActiveTree());
 
-  DCHECK(std::find(layers_with_copy_output_request_.begin(),
-                   layers_with_copy_output_request_.end(),
-                   layer) == layers_with_copy_output_request_.end());
+  if (std::find(layers_with_copy_output_request_.begin(),
+                layers_with_copy_output_request_.end(),
+                layer) != layers_with_copy_output_request_.end())
+    return;
   layers_with_copy_output_request_.push_back(layer);
 }