[FlexNG] Set margins on item's LayoutBox before computing position

FlexibleBoxAlgorithm reads margins from LayoutBox. We weren't
positioning items with margins correctly in FlexNG before this patch.

Bug: 845235
Change-Id: I4d05fd6107f0140d69c6aee3f56d78dc430862c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913707Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714813}

third_party/blink/renderer/core/layout/ng/ng_flex_layout_algorithm.cc

@@ -319,6 +319,9 @@ void NGFlexLayoutAlgorithm::ConstructAndAppendFlexItems() {
 
     NGPhysicalBoxStrut physical_child_margins =
         ComputePhysicalMargins(child_space, child_style);
+    // Set margin because FlexibleBoxAlgorithm reads it from legacy.
+    child.GetLayoutBox()->SetMargin(physical_child_margins);
+
     LayoutUnit main_axis_margin = is_horizontal_flow_
                                       ? physical_child_margins.HorizontalSum()
                                       : physical_child_margins.VerticalSum();

third_party/blink/web_tests/TestExpectations

@@ -1384,7 +1384,6 @@ crbug.com/591099 external/wpt/css/css-flexbox/flexbox_align-items-stretch-3.html
 crbug.com/591099 virtual/layout_ng_flex_box/css3/flexbox/auto-height-column-with-border-and-padding.html [ Failure ]
 crbug.com/591099 virtual/layout_ng_flex_box/css3/flexbox/flex-align-vertical-writing-mode.html [ Failure ]
 crbug.com/591099 virtual/layout_ng_flex_box/css3/flexbox/flex-align.html [ Failure ]
-crbug.com/591099 virtual/layout_ng_flex_box/css3/flexbox/flex-flow-margins-auto-size.html [ Failure ]
 crbug.com/591099 virtual/layout_ng_flex_box/css3/flexbox/flex-order.html [ Failure ]
 crbug.com/591099 virtual/layout_ng_flex_box/css3/flexbox/flexbox-baseline-margins.html [ Failure ]
 crbug.com/591099 virtual/layout_ng_flex_box/css3/flexbox/flexbox-baseline.html [ Failure ]