Ensure transform nodes do not need a local update if the tree is clean

There are two dirty bits for whether TransformTree::UpdateLocalTransform
needs to be called: TransformNode::needs_local_transform_update, and
TransformTree::needs_update. If the transform tree does not need an
update, we skip calling TransformTree::UpdateLocalTransform.

This patch DCHECKS that no transform node needs an update if we are
skipping TransformTree::UpdateLocalTransform due to the tree not needing
an update.

Bug: 864567
Change-Id: Ib43ce23d8715c1cf802aaefb8e4068ce24fd07b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1650120
Auto-Submit: Philip Rogers <pdr@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667326}

cc/test/layer_tree_pixel_test.cc

@@ -284,8 +284,11 @@ void LayerTreePixelTest::InitializeForLayerListMode(
   ScrollNode scroll_node;
   property_trees->scroll_tree.Insert(scroll_node, 0);
 
-  TransformNode transform_node;
-  property_trees->transform_tree.Insert(transform_node, 0);
+  TransformTree& transform_tree = property_trees->transform_tree;
+  auto& transform_node =
+      *transform_tree.Node(transform_tree.Insert(TransformNode(), 0));
+  transform_node.source_node_id = transform_node.parent_id;
+  transform_tree.set_needs_update(true);
 
   *root_layer = Layer::Create();
   (*root_layer)->SetBounds(gfx::Size(100, 100));

cc/trees/draw_property_utils.cc

@@ -838,8 +838,17 @@ void FindLayersThatNeedUpdates(LayerTreeImpl* layer_tree_impl,
 }
 
 void ComputeTransforms(TransformTree* transform_tree) {
-  if (!transform_tree->needs_update())
+  if (!transform_tree->needs_update()) {
+#if DCHECK_IS_ON()
+    // If the transform tree does not need an update, no TransformNode should
+    // need a local transform update.
+    for (int i = TransformTree::kContentsRootNodeId;
+         i < static_cast<int>(transform_tree->size()); ++i) {
+      DCHECK(!transform_tree->Node(i)->needs_local_transform_update);
+    }
+#endif
     return;
+  }
   for (int i = TransformTree::kContentsRootNodeId;
        i < static_cast<int>(transform_tree->size()); ++i)
     transform_tree->UpdateTransforms(i);

cc/trees/layer_tree_host_pixeltest_masks.cc

@@ -450,10 +450,12 @@ TEST_P(LayerTreeHostLayerListPixelTestNonSkia, ScaledMaskWithEffect) {
 
   // Scale the mask with a non-integral transform. This will trigger the
   // AA path in the renderer.
-  TransformNode transform;
-  transform.local = gfx::Transform();
-  transform.local.Scale(1.5, 1.5);
-  property_trees.transform_tree.Insert(transform, 1);
+  TransformTree& transform_tree = property_trees.transform_tree;
+  auto& transform_node =
+      *transform_tree.Node(transform_tree.Insert(TransformNode(), 1));
+  transform_node.source_node_id = transform_node.parent_id;
+  transform_node.local = gfx::Transform();
+  transform_node.local.Scale(1.5, 1.5);
 
   scoped_refptr<SolidColorLayer> background =
       CreateSolidColorLayer(gfx::Rect(100, 100), SK_ColorWHITE);

cc/trees/property_tree.cc

@@ -233,6 +233,8 @@ void TransformTree::UpdateTransforms(int id) {
   UpdateNodeAndAncestorsHaveIntegerTranslations(node, parent_node);
   UpdateTransformChanged(node, parent_node, source_node);
   UpdateNodeAndAncestorsAreAnimatedOrInvertible(node, parent_node);
+
+  DCHECK(!node->needs_local_transform_update);
 }
 
 bool TransformTree::IsDescendant(int desc_id, int source_id) const {

cc/trees/transform_node.h

@@ -67,7 +67,8 @@ struct CC_EXPORT TransformNode {
   // context.
   int sorting_context_id;
 
-  // TODO(vollick): will be moved when accelerated effects are implemented.
+  // True if |TransformTree::UpdateLocalTransform| needs to be called which
+  // will update |to_parent| and |source_to_parent| (if possible).
   bool needs_local_transform_update : 1;
 
   // Whether this node or any ancestor has a potentially running