Fix double subtraction of sent_delta on sync tree activation.

cc/layers/layer_impl.cc

 // Copyright 2012 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_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "base/debug/trace_event_argument.h"
 #include "base/json/json_reader.h"
 #include "base/strings/stringprintf.h"
@@ skipping 532 matching lines @@
       is_container_for_fixed_position_layers_);
   layer->SetPositionConstraint(position_constraint_);
   layer->SetShouldFlattenTransform(should_flatten_transform_);
   layer->SetUseParentBackfaceVisibility(use_parent_backface_visibility_);
   layer->SetTransformAndInvertibility(transform_, transform_is_invertible_);
 
   layer->SetScrollClipLayer(scroll_clip_layer_ ? scroll_clip_layer_->id()
                                                : Layer::INVALID_ID);
   layer->set_user_scrollable_horizontal(user_scrollable_horizontal_);
   layer->set_user_scrollable_vertical(user_scrollable_vertical_);
-  layer->SetScrollOffsetAndDelta(
-      scroll_offset_,
-      layer->ScrollDelta() - layer->sent_scroll_delta());
+
+  // Save the difference but clear the sent delta so that we don't subtract
+  // it again in SetScrollOffsetAndDelta's pending twin mirroring logic.
+  gfx::Vector2dF remaining_delta =
+      layer->ScrollDelta() - layer->sent_scroll_delta();
   layer->SetSentScrollDelta(gfx::Vector2dF());
+  layer->SetScrollOffsetAndDelta(scroll_offset_, remaining_delta);
+
   layer->Set3dSortingContextId(sorting_context_id_);
   layer->SetNumDescendantsThatDrawContent(num_descendants_that_draw_content_);
 
   LayerImpl* scroll_parent = nullptr;
   if (scroll_parent_) {
     scroll_parent = layer->layer_tree_impl()->LayerById(scroll_parent_->id());
     DCHECK(scroll_parent);
   }
 
   layer->SetScrollParent(scroll_parent);
@@ skipping 1022 matching lines @@
 
 void LayerImpl::NotifyAnimationFinished(
     base::TimeTicks monotonic_time,
     Animation::TargetProperty target_property,
     int group) {
   if (target_property == Animation::ScrollOffset)
     layer_tree_impl_->InputScrollAnimationFinished();
 }
 
 }  // namespace cc