Rewrite Layer::SetScrollOffset{FromImplSide} property tree fast-path

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 758 matching lines @@
 void Layer::SetScrollOffset(const gfx::ScrollOffset& scroll_offset) {
   DCHECK(IsPropertyChangeAllowed());
 
   if (inputs_.scroll_offset == scroll_offset)
     return;
   inputs_.scroll_offset = scroll_offset;
 
   if (!layer_tree_host_)
     return;
 
-  PropertyTrees* property_trees = layer_tree_host_->property_trees();
-  if (scroll_tree_index() != ScrollTree::kInvalidNodeId && scrollable())
-    property_trees->scroll_tree.SetScrollOffset(id(), scroll_offset);
-
-  if (TransformNode* transform_node =
-          property_trees->transform_tree.UpdateNodeFromOwningLayerId(id())) {
-    DCHECK_EQ(transform_tree_index(), transform_node->id);
-    transform_node->scroll_offset = CurrentScrollOffset();
-    transform_node->needs_local_transform_update = true;
-    property_trees->transform_tree.set_needs_update(true);
-  }
+  UpdateScrollOffset(scroll_offset);
 
   SetNeedsCommit();
 }
 
 void Layer::SetScrollOffsetFromImplSide(
     const gfx::ScrollOffset& scroll_offset) {
   DCHECK(IsPropertyChangeAllowed());
   // This function only gets called during a BeginMainFrame, so there
   // is no need to call SetNeedsUpdate here.
   DCHECK(layer_tree_host_ && layer_tree_host_->CommitRequested());
   if (inputs_.scroll_offset == scroll_offset)
     return;
   inputs_.scroll_offset = scroll_offset;
   SetNeedsPushProperties();
 
-  PropertyTrees* property_trees = layer_tree_host_->property_trees();
-  if (scroll_tree_index() != ScrollTree::kInvalidNodeId && scrollable())
-    property_trees->scroll_tree.SetScrollOffset(id(), scroll_offset);
-
-  if (TransformNode* transform_node =
-          property_trees->transform_tree.UpdateNodeFromOwningLayerId(id())) {
-    DCHECK_EQ(transform_tree_index(), transform_node->id);
-    transform_node->scroll_offset = CurrentScrollOffset();
-    transform_node->needs_local_transform_update = true;
-    property_trees->transform_tree.set_needs_update(true);
-  }
+  UpdateScrollOffset(scroll_offset);
 
   if (!inputs_.did_scroll_callback.is_null())
     inputs_.did_scroll_callback.Run(scroll_offset);
 
   // The callback could potentially change the layer structure:
   // "this" may have been destroyed during the process.
 }
 
+void Layer::UpdateScrollOffset(const gfx::ScrollOffset& scroll_offset) {
+  DCHECK(scrollable());
+  if (scroll_tree_index() == ScrollTree::kInvalidNodeId) {
+    // Ensure the property trees just have not been built yet but are marked for
+    // being built which will set the correct scroll offset values.
+    DCHECK(layer_tree_host_->property_trees()->needs_rebuild);
+    return;
+  }
+
+  // If a scroll node exists, it should have an associated transform node.
+  DCHECK(transform_tree_index() != TransformTree::kInvalidNodeId);
+
+  auto& property_trees = *layer_tree_host_->property_trees();
+  property_trees.scroll_tree.SetScrollOffset(id(), scroll_offset);
+  auto* transform_node =
+      property_trees.transform_tree.Node(transform_tree_index());
+  DCHECK_EQ(transform_tree_index(), transform_node->id);
+  transform_node->scroll_offset = CurrentScrollOffset();
+  transform_node->needs_local_transform_update = true;
+  property_trees.transform_tree.set_needs_update(true);
+}
+
 void Layer::SetScrollClipLayerId(int clip_layer_id) {
   DCHECK(IsPropertyChangeAllowed());
   if (inputs_.scroll_clip_layer_id == clip_layer_id)
     return;
   inputs_.scroll_clip_layer_id = clip_layer_id;
   SetPropertyTreesNeedRebuild();
   SetNeedsCommit();
 }
 
 Layer* Layer::scroll_clip_layer() const {
@@ skipping 607 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