blink: add support for box-ordinal for LayoutFlexibleBox

This provides mapping in ComputedStyle for box-ordinal.

BUG=993813
TEST=none

Change-Id: I0121a52d3c5ace231e36528f9513afbf39341a4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761404Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688655}

third_party/blink/renderer/core/layout/order_iterator.cc

@@ -61,7 +61,7 @@ LayoutBox* OrderIterator::Next() {
       current_child_ = current_child_->NextSiblingBox();
     }
   } while (!current_child_ ||
-           current_child_->StyleRef().Order() != *order_values_iterator_);
+           ResolvedOrder(*current_child_) != *order_values_iterator_);
 
   return current_child_;
 }
@@ -72,12 +72,20 @@ void OrderIterator::Reset() {
   is_reset_ = true;
 }
 
+int OrderIterator::ResolvedOrder(const LayoutBox& child) const {
+  if (container_box_->StyleRef().Display() == EDisplay::kWebkitBox ||
+      container_box_->StyleRef().Display() == EDisplay::kWebkitInlineBox) {
+    return child.StyleRef().BoxOrdinalGroup();
+  }
+  return child.StyleRef().Order();
+}
+
 OrderIteratorPopulator::~OrderIteratorPopulator() {
   iterator_.Reset();
 }
 
 void OrderIteratorPopulator::CollectChild(const LayoutBox* child) {
-  iterator_.order_values_.insert(child->StyleRef().Order());
+  iterator_.order_values_.insert(iterator_.ResolvedOrder(*child));
 }
 
 }  // namespace blink

third_party/blink/renderer/core/layout/order_iterator.h

@@ -62,6 +62,9 @@ class OrderIterator {
  private:
   void Reset();
 
+  // Returns the order to use for |child|.
+  int ResolvedOrder(const LayoutBox& child) const;
+
   const LayoutBox* container_box_;
 
   LayoutBox* current_child_ = nullptr;