Revert of Add DCHECK for duplicate layers with the same cc::ElementId.

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 7957734ff26fe535b21b81196117f4bc7881f588..4cd9aae75103100b01bf11715f3d23ce8405f9f7 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -544,25 +544,18 @@
 }
 
 void LayerTreeImpl::AddToElementMap(LayerImpl* layer) {
-  ElementId element_id = layer->element_id();
-  if (!element_id)
+  if (!layer->element_id())
     return;
 
   TRACE_EVENT2(TRACE_DISABLED_BY_DEFAULT("compositor-worker"),
                "LayerTreeImpl::AddToElementMap", "element",
-               element_id.AsValue().release(), "layer_id", layer->id());
-
-#if DCHECK_IS_ON()
-  LayerImpl* existing_layer = LayerByElementId(element_id);
-  bool element_id_collision_detected =
-      existing_layer && existing_layer != layer;
-  DCHECK(!element_id_collision_detected);
-#endif
-
-  element_layers_map_[element_id] = layer->id();
+               layer->element_id().AsValue().release(), "layer_id",
+               layer->id());
+
+  element_layers_map_[layer->element_id()] = layer->id();
 
   layer_tree_host_impl_->mutator_host()->RegisterElement(
-      element_id,
+      layer->element_id(),
       IsActiveTree() ? ElementListType::ACTIVE : ElementListType::PENDING);
 }