Make fast/multicol/mixed-positioning-stacking-order.html valid.

The test assumed that block fragmentation takes place after relative
positioning, but it's the other way around:
https://www.w3.org/TR/css-break-3/#transforms

Bug: 829028
Change-Id: I297bf572354216188b5a8c8d007f7a137c36e773
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2554543Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830212}

third_party/blink/web_tests/TestExpectations

@@ -1098,7 +1098,6 @@ crbug.com/1066626 virtual/layout_ng_block_frag/fast/multicol/layers-split-across
 crbug.com/829028 virtual/layout_ng_block_frag/fast/multicol/line-too-tall-for-second-outer-row.html [ Failure ]
 crbug.com/1066626 virtual/layout_ng_block_frag/fast/multicol/mixed-opacity-fixed-test.html [ Failure ]
 crbug.com/1079031 virtual/layout_ng_block_frag/fast/multicol/mixed-opacity-test.html [ Crash Failure ]
-crbug.com/1079031 virtual/layout_ng_block_frag/fast/multicol/mixed-positioning-stacking-order.html [ Failure ]
 crbug.com/829028 virtual/layout_ng_block_frag/fast/multicol/nested-auto-height-short-first-row.html [ Failure ]
 crbug.com/829028 virtual/layout_ng_block_frag/fast/multicol/nested-balanced-inner-column-count-1-with-forced-break.html [ Failure ]
 crbug.com/829028 virtual/layout_ng_block_frag/fast/multicol/nested-columns.html [ Failure ]

third_party/blink/web_tests/fast/multicol/mixed-positioning-stacking-order.html

@@ -9,7 +9,7 @@
 <div style="height:200px"></div>
 <div style="width:100px;height:100px;background-color:purple;">3</div>
 </div>
-<div style="position:relative; z-index:1; background-color:lightblue; width:100%; height:200px; top:100px">1</div>
+<div style="position:absolute; z-index:1; background-color:lightblue; width:100%; height:200px; top:100px">1</div>
 
 <div style="position:fixed;background-color:gold;left:68px;top:13px;height:200px;width:100px">2</div>
 </div>