[GridNG] Fix center alignment.

Center alignment is based on the margin-box. AlignmentOffset was
returning the offset to the margin-box instead of to the border-box.

This patch adds "margin_start" to the calculation correcting this
mistake. This was uncovered after:
https://chromium-review.googlesource.com/c/chromium/src/+/2569848
landed.

Bug: 1045599
Change-Id: If0c720ddae9a910ef2c08ee8efff22e8c2057982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575502Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834285}

third_party/blink/renderer/core/layout/ng/grid/ng_grid_layout_algorithm.cc

@@ -1256,7 +1256,7 @@ LayoutUnit AlignmentOffset(LayoutUnit container_size,
     case AxisEdge::kStart:
       return margin_start;
     case AxisEdge::kCenter:
-      return (container_size - size - margin_start - margin_end) / 2;
+      return (container_size - size + margin_start - margin_end) / 2;
     case AxisEdge::kEnd:
       return container_size - margin_end - size;
     case AxisEdge::kBaseline:

third_party/blink/web_tests/TestExpectations

@@ -3447,9 +3447,6 @@ crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/gri
 crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-align-content-vertical-lr.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-align-content-vertical-rl.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-align-content.html [ Failure ]
-crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-align-justify-margin-border-padding-vertical-lr.html [ Failure ]
-crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-align-justify-margin-border-padding-vertical-rl.html [ Failure ]
-crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-align-justify-margin-border-padding.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-align-justify-overflow.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-alignment-implies-size-change-029.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/external/wpt/css/css-grid/alignment/grid-alignment-implies-size-change-030.html [ Failure ]
@@ -3941,7 +3938,6 @@ crbug.com/1045599 virtual/layout-ng-grid/fast/css-grid-layout/grid-initialize-sp
 crbug.com/1045599 virtual/layout-ng-grid/fast/css-grid-layout/grid-intrinsic-maximums.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/fast/css-grid-layout/grid-item-addition-auto-placement-update.html [ Failure Crash ]
 crbug.com/1045599 virtual/layout-ng-grid/fast/css-grid-layout/grid-item-addition-track-breadth-update.html [ Failure ]
-crbug.com/1045599 virtual/layout-ng-grid/fast/css-grid-layout/grid-item-auto-sized-align-justify-margin-border-padding.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/fast/css-grid-layout/grid-item-grid-container-percentage-rows.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/fast/css-grid-layout/grid-item-multiple-minmax-content-resolution.html [ Failure ]
 crbug.com/1045599 virtual/layout-ng-grid/fast/css-grid-layout/grid-item-named-grid-line-resolution.html [ Failure ]