Update from https://crrev.com/320931

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 <algorithm>
 
 #include "base/atomic_sequence_num.h"
 #include "base/location.h"
@@ skipping 34 matching lines @@
       layer_id_(g_next_layer_id.GetNext() + 1),
       ignore_set_needs_commit_(false),
       sorting_context_id_(0),
       parent_(nullptr),
       layer_tree_host_(nullptr),
       scroll_clip_layer_id_(INVALID_ID),
       num_descendants_that_draw_content_(0),
       transform_tree_index_(-1),
       opacity_tree_index_(-1),
       clip_tree_index_(-1),
+      should_flatten_transform_from_property_tree_(false),
       should_scroll_on_main_thread_(false),
       have_wheel_event_handlers_(false),
       have_scroll_event_handlers_(false),
       user_scrollable_horizontal_(true),
       user_scrollable_vertical_(true),
       is_root_for_isolated_group_(false),
       is_container_for_fixed_position_layers_(false),
       is_drawable_(false),
       draws_content_(false),
       hide_layer_and_subtree_(false),
@@ skipping 1222 matching lines @@
   return false;
 }
 
 gfx::Transform Layer::screen_space_transform_from_property_trees(
     const TransformTree& tree) const {
   gfx::Transform xform(1, 0, 0, 1, offset_to_transform_parent().x(),
                        offset_to_transform_parent().y());
   if (transform_tree_index() >= 0) {
     gfx::Transform ssxform = tree.Node(transform_tree_index())->data.to_screen;
     xform.ConcatTransform(ssxform);
+    if (should_flatten_transform_from_property_tree_)
+      xform.FlattenTo2d();
   }
   xform.Scale(1.0 / contents_scale_x(), 1.0 / contents_scale_y());
   return xform;
 }
 
 gfx::Transform Layer::draw_transform_from_property_trees(
     const TransformTree& tree) const {
   const TransformNode* node = tree.Node(transform_tree_index());
   // TODO(vollick): ultimately we'll need to find this information (whether or
   // not we establish a render surface) somewhere other than the layer.
   const TransformNode* target_node =
       has_render_surface_ ? node : tree.Node(node->data.content_target_id);
 
   gfx::Transform xform;
   const bool owns_non_root_surface = parent() && render_surface();
   if (!owns_non_root_surface) {
     // If you're not the root, or you don't own a surface, you need to apply
     // your local offset.
     xform = node->data.to_target;
+    if (should_flatten_transform_from_property_tree_)
+      xform.FlattenTo2d();
     xform.Translate(offset_to_transform_parent().x(),
                     offset_to_transform_parent().y());
   } else {
     // Surfaces need to apply their sublayer scale.
     xform.Scale(target_node->data.sublayer_scale.x(),
                 target_node->data.sublayer_scale.y());
   }
   xform.Scale(1.0 / contents_scale_x(), 1.0 / contents_scale_y());
   return xform;
 }
@@ skipping 17 matching lines @@
 }
 
 void Layer::SetFrameTimingRequests(
     const std::vector<FrameTimingRequest>& requests) {
   frame_timing_requests_ = requests;
   frame_timing_requests_dirty_ = true;
   SetNeedsCommit();
 }
 
 }  // namespace cc