Fix RemoveFromScrollTree and RemoveFromClipTree

cc/trees/tree_synchronizer.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/tree_synchronizer.h"
 
 #include <set>
 
 #include "base/containers/hash_tables.h"
 #include "base/containers/scoped_ptr_hash_map.h"
@@ skipping 224 matching lines @@
                             layer->needs_push_properties();
   *num_dependents_need_push_properties_for_parent += add_self_to_parent ? 1 : 0;
 }
 
 static void CheckScrollAndClipPointersRecursive(Layer* layer,
                                                 LayerImpl* layer_impl) {
   DCHECK_EQ(!!layer, !!layer_impl);
   if (!layer)
     return;
 
-  DCHECK_EQ(!!layer->scroll_parent(), !!layer_impl->scroll_parent());
-  if (layer->scroll_parent())
+  // Having a scroll parent on the impl thread implies having one the main
+  // thread, too. The main thread may have a scroll parent that is not in the
+  // tree because it's been removed but not deleted. In this case, the layer
+  // impl will have no scroll parent. Same argument applies for clip parents and
+  // scroll/clip children.
+  DCHECK(!layer_impl->scroll_parent() || !!layer->scroll_parent());
+  DCHECK(!layer_impl->clip_parent() || !!layer->clip_parent());
+  DCHECK(!layer_impl->scroll_children() || !!layer->scroll_children());
+  DCHECK(!layer_impl->clip_children() || !!layer->clip_children());
+
+  if (layer_impl->scroll_parent())
     DCHECK_EQ(layer->scroll_parent()->id(), layer_impl->scroll_parent()->id());
 
-  DCHECK_EQ(!!layer->clip_parent(), !!layer_impl->clip_parent());
-  if (layer->clip_parent())
+  if (layer_impl->clip_parent())
     DCHECK_EQ(layer->clip_parent()->id(), layer_impl->clip_parent()->id());
 
-  DCHECK_EQ(!!layer->scroll_children(), !!layer_impl->scroll_children());
-  if (layer->scroll_children()) {
+  if (layer_impl->scroll_children()) {
     for (std::set<Layer*>::iterator it = layer->scroll_children()->begin();
          it != layer->scroll_children()->end();
          ++it) {
       DCHECK_EQ((*it)->scroll_parent(), layer);
     }
     for (std::set<LayerImpl*>::iterator it =
              layer_impl->scroll_children()->begin();
          it != layer_impl->scroll_children()->end();
          ++it) {
       DCHECK_EQ((*it)->scroll_parent(), layer_impl);
     }
   }
 
-  DCHECK_EQ(!!layer->clip_children(), !!layer_impl->clip_children());
-  if (layer->clip_children()) {
+  if (layer_impl->clip_children()) {
     for (std::set<Layer*>::iterator it = layer->clip_children()->begin();
          it != layer->clip_children()->end();
          ++it) {
       DCHECK_EQ((*it)->clip_parent(), layer);
     }
     for (std::set<LayerImpl*>::iterator it =
              layer_impl->clip_children()->begin();
          it != layer_impl->clip_children()->end();
          ++it) {
       DCHECK_EQ((*it)->clip_parent(), layer_impl);
@@ skipping 16 matching lines @@
 #endif
 }
 
 void TreeSynchronizer::PushProperties(LayerImpl* layer, LayerImpl* layer_impl) {
   size_t num_dependents_need_push_properties = 0;
   PushPropertiesInternal(
       layer, layer_impl, &num_dependents_need_push_properties);
 }
 
 }  // namespace cc