Support bottom-right anchored fixed-position elements during a pinch gesture

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 1ff75b39139762b14c1a1115d2e87d5c412cf2c6..90a2440f28c7de5cfebab2a5b10a20f414ebc6da 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -216,6 +216,15 @@ void LayerTreeImpl::UpdateMaxScrollOffset() {
   max_scroll.ClampToMin(gfx::Vector2dF());
 
   root_scroll_layer_->SetMaxScrollOffset(gfx::ToFlooredVector2d(max_scroll));
+
+  gfx::Vector2dF scrollable_viewport_size =
+      gfx::RectF(ScrollableViewportSize()).bottom_right() - gfx::PointF();
+  gfx::Vector2dF original_scrollable_viewport_size =
+      gfx::RectF(layer_tree_host_impl_->VisibleViewportSize()).bottom_right() -
+      gfx::PointF();
+  original_scrollable_viewport_size.Scale(1 / page_scale_factor());
+  root_scroll_layer_->SetFixedContainerSizeDelta(
+      scrollable_viewport_size - original_scrollable_viewport_size);
 }
 
 gfx::Transform LayerTreeImpl::ImplTransform() const {