Fix double subtraction of sent_delta on sync tree activation.

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index 2b1193370eee237e307ce28f4074ce6a3134ce8c..db581d24ae717de58e906e82b4f5051f3eb9786d 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -550,10 +550,14 @@ void LayerImpl::PushPropertiesTo(LayerImpl* layer) {
                                                : 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_);