[layoutng] Add out-of-flow objects to the positioned objects map

This fixes the main remaining assertion in legacy/ng interop.

BUG=635619
R=eae@chromium.org,ikilpatrick@chromium.org,glebl@chromium.org

Review-Url: https://codereview.chromium.org/2347283002
Cr-Commit-Position: refs/heads/master@{#419311}

third_party/WebKit/Source/core/layout/ng/ng_box.cc

@@ -26,6 +26,8 @@ NGBox::NGBox(ComputedStyle* style) : layout_box_(nullptr), style_(style) {
 
 bool NGBox::Layout(const NGConstraintSpace* constraint_space,
                    NGFragment** out) {
+  if (layout_box_ && layout_box_->isOutOfFlowPositioned())
+    layout_box_->containingBlock()->insertPositionedObject(layout_box_);
   // We can either use the new layout code to do the layout and then copy the
   // resulting size to the LayoutObject, or use the old layout code and
   // synthesize a fragment.