Harmonize LayerTreeHost/LayerTreeHostImpl synchronization steps

cc/layers/layer.cc

 // Copyright 2010 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/layers/layer.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 1165 matching lines @@
   layer->SetUseParentBackfaceVisibility(inputs_.use_parent_backface_visibility);
   layer->SetUseLocalTransformForBackfaceVisibility(
       use_local_transform_for_backface_visibility_);
   layer->SetShouldCheckBackfaceVisibility(should_check_backface_visibility_);
 
   layer->SetScrollClipLayer(inputs_.scroll_clip_layer_id);
   layer->set_user_scrollable_horizontal(inputs_.user_scrollable_horizontal);
   layer->set_user_scrollable_vertical(inputs_.user_scrollable_vertical);
   layer->SetMutableProperties(inputs_.mutable_properties);
 
+  // The property trees must be safe to access because they will be used below
+  // to call |SetScrollOffsetClobberActiveValue|.
+  DCHECK(layer->layer_tree_impl()->PropertyTreeSyncComplete());
+
   // When a scroll offset animation is interrupted the new scroll position on
   // the pending tree will clobber any impl-side scrolling occuring on the
   // active tree. To do so, avoid scrolling the pending tree along with it
   // instead of trying to undo that scrolling later.
   if (ScrollOffsetAnimationWasInterrupted())
     layer->layer_tree_impl()
         ->property_trees()
         ->scroll_tree.SetScrollOffsetClobberActiveValue(layer->id());
 
   if (needs_show_scrollbars_)
@@ skipping 245 matching lines @@
       ->subtree_has_copy_request;
 }
 
 gfx::Transform Layer::ScreenSpaceTransform() const {
   DCHECK_NE(transform_tree_index_, TransformTree::kInvalidNodeId);
   return draw_property_utils::ScreenSpaceTransform(
       this, layer_tree_host_->property_trees()->transform_tree);
 }
 
 }  // namespace cc