cc: Add CHECKs for diagnosing invalid property tree indices post-commit.

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 390e14e743f1d89b9e7e2c2de3ecd87c26cc9564..7e1e56ae8138f373dcaf033a58e47cb6d450cde5 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -63,6 +63,29 @@ static base::StaticAtomicSequenceNumber s_layer_tree_host_sequence_number;
 }
 
 namespace cc {
+namespace {
+
+void EnsureValidIndicesOnLayer(Layer* layer, PropertyTrees* property_trees) {
+  CHECK_EQ(property_trees->sequence_number,
+           layer->property_tree_sequence_number());
+  CHECK(property_trees->transform_tree.Node(layer->transform_tree_index()));
+  CHECK(property_trees->clip_tree.Node(layer->clip_tree_index()));
+  CHECK(property_trees->effect_tree.Node(layer->effect_tree_index()));
+  CHECK(property_trees->scroll_tree.Node(layer->scroll_tree_index()));
+}
+
+void EnsureValidPropertyTreeState(LayerTreeHost* host) {
+  // Temporary check to debug crbug.com/726423. The property tree indices on
+  // the LayerTreeImpl should be valid after all state synchronization has
+  // finished.
+  for (auto* layer : *host)
+    EnsureValidIndicesOnLayer(layer, host->property_trees());
+
+  for (auto id : host->property_trees()->effect_tree.mask_layer_ids())
+    EnsureValidIndicesOnLayer(host->LayerById(id), host->property_trees());
+}
+
+}  // namespace
 
 LayerTreeHost::InitParams::InitParams() {}
 
@@ -304,6 +327,8 @@ void LayerTreeHost::FinishCommitOnImplThread(
 
   sync_tree->set_source_frame_number(SourceFrameNumber());
 
+  EnsureValidPropertyTreeState(this);
+
   if (needs_full_tree_sync_)
     TreeSynchronizer::SynchronizeTrees(root_layer(), sync_tree);
 
@@ -745,21 +770,13 @@ bool LayerTreeHost::DoUpdateLayers(Layer* root_layer) {
           property_trees->AsTracedValue());
     }
 
-    // Temporary check to debug crbug.com/726423. The property tree indices on
-    // the LayerTreeImpl should be valid after all state synchronization has
-    // finished.
-    for (auto* layer : *this) {
-      CHECK(property_trees_.transform_tree.Node(layer->transform_tree_index()));
-      CHECK(property_trees_.clip_tree.Node(layer->clip_tree_index()));
-      CHECK(property_trees_.effect_tree.Node(layer->effect_tree_index()));
-      CHECK(property_trees_.scroll_tree.Node(layer->scroll_tree_index()));
-    }
-
     draw_property_utils::UpdatePropertyTrees(this, property_trees);
     draw_property_utils::FindLayersThatNeedUpdates(this, property_trees,
                                                    &update_layer_list);
   }
 
+  EnsureValidPropertyTreeState(this);
+
   bool content_is_suitable_for_gpu = true;
   bool did_paint_content =
       PaintContent(update_layer_list, &content_is_suitable_for_gpu);