[LayoutNG] Re-enable NGOutOfFlowLayoutPart unit test

It looks like the bug behind the disabled unit test
NGOutOfFlowLayoutPartTest.PositionedFragmentationWithBottomProperty
got fixed without us noticing it. Probably got fixed in CL:2422705.

Re-enabling the unit test.

Bug: 1115584
Change-Id: I55dcb0f08dce33a50f262b5ae74ade9da28e8359
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487914
Reviewed-by: Morten Stenshorne <[email protected]>
Reviewed-by: Alison Maher <[email protected]>
Commit-Queue: Benjamin Beaudry <[email protected]>
Cr-Commit-Position: refs/heads/master@{#819178}

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

@@ -613,9 +613,7 @@ TEST_F(
   EXPECT_EQ(expectation, dump);
 }
 
-// TODO(bebeaudr): Enable when http://crbug.com/1115584 is fixed.
-TEST_F(NGOutOfFlowLayoutPartTest,
-       DISABLED_PositionedFragmentationWithBottomProperty) {
+TEST_F(NGOutOfFlowLayoutPartTest, PositionedFragmentationWithBottomProperty) {
   SetBodyInnerHTML(
       R"HTML(
       <style>