[Reland] Add DCHECK for duplicate layers with the same cc::ElementId.

cc/trees/layer_tree_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 526 matching lines @@
     return Layer::INVALID_ID;
 
   return iter->second;
 }
 
 LayerImpl* LayerTreeImpl::LayerByElementId(ElementId element_id) const {
   return LayerById(LayerIdByElementId(element_id));
 }
 
 void LayerTreeImpl::AddToElementMap(LayerImpl* layer) {
-  if (!layer->element_id())
+  ElementId element_id = layer->element_id();
+  if (!element_id)
     return;
 
   TRACE_EVENT2(TRACE_DISABLED_BY_DEFAULT("compositor-worker"),
                "LayerTreeImpl::AddToElementMap", "element",
-               layer->element_id().AsValue().release(), "layer_id",
-               layer->id());
+               element_id.AsValue().release(), "layer_id", layer->id());
 
-  element_layers_map_[layer->element_id()] = layer->id();
+#if DCHECK_IS_ON()
+  LayerImpl* existing_layer = LayerByElementId(element_id);
+  bool element_id_collision_detected =
+      existing_layer != nullptr && existing_layer != layer;

[ajuma, 2017/01/18 14:22:52]

existing_layer && existing_layer != layer;
might be more readable.

[skobes, 2017/01/18 16:24:08]

Done.

+  DCHECK(!element_id_collision_detected);
+#endif
+
+  element_layers_map_[element_id] = layer->id();
 
   layer_tree_host_impl_->mutator_host()->RegisterElement(
-      layer->element_id(),
+      element_id,
       IsActiveTree() ? ElementListType::ACTIVE : ElementListType::PENDING);
 }
 
 void LayerTreeImpl::RemoveFromElementMap(LayerImpl* layer) {
   if (!layer->element_id())
     return;
 
   TRACE_EVENT2(TRACE_DISABLED_BY_DEFAULT("compositor-worker"),
                "LayerTreeImpl::RemoveFromElementMap", "element",
                layer->element_id().AsValue().release(), "layer_id",
@@ skipping 1528 matching lines @@
 
 void LayerTreeImpl::ResetAllChangeTracking() {
   layers_that_should_push_properties_.clear();
   // Iterate over all layers, including masks.
   for (auto& layer : *layers_)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking();
 }
 
 }  // namespace cc