css-flexbox: Fix static position for out-of-flow webkit-box

Specifically, for positioned elements in a webkit-box with
a column orientation. This matches similar code in
LayoutDeprecatedFlexibleBox::LayoutVerticalBox.

BUG=993813
TEST=fast/deprecated-flexbox/012.html

Change-Id: Ibad9868a077d3a1ff62f2b2844cb086b78c33612
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809526
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698982}

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

@@ -1304,9 +1304,12 @@ LayoutUnit LayoutFlexibleBox::StaticCrossAxisPositionForPositionedChild(
 
 LayoutUnit LayoutFlexibleBox::StaticInlinePositionForPositionedChild(
     const LayoutBox& child) {
-  return StartOffsetForContent() +
-         (IsColumnFlow() ? StaticCrossAxisPositionForPositionedChild(child)
-                         : StaticMainAxisPositionForPositionedChild(child));
+  const LayoutUnit start_offset = StartOffsetForContent();
+  if (IsColumnFlow() && StyleRef().IsDeprecatedWebkitBox())
+    return start_offset;
+  return start_offset + (IsColumnFlow()
+                             ? StaticCrossAxisPositionForPositionedChild(child)
+                             : StaticMainAxisPositionForPositionedChild(child));
 }
 
 LayoutUnit LayoutFlexibleBox::StaticBlockPositionForPositionedChild(