[gap-decorations] Address remaining flex test failures

One test failure was actually a incorrect reference file, we already had
the correct behavior, which is that a column rule in a column flex box
stretches until the end of the fragment/container.

The other was a genuine bug, in flex fragmentation we were using the
item offsets rather than the line offsets for the `line_cross_end`.
While this works for most cases, cases with `align-items` introduce
discrepancies between the item offsets and the line offsets. Moreover,
the ref file was also incorrect.

Bug: 440123087
Change-Id: I5cc0670ea82736acf35aa31f8d421197bf638420
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7804568
Commit-Queue: Javier Contreras <javiercon@microsoft.com>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1623263}

Author: Javier Contreras

css/css-gaps/flex/fragmentation/flex-gap-decorations-fragmentation-005-ref.html

@@ -39,13 +39,12 @@
     }
 </style>
 
+<div style="left: 175px; height: 10px; width: 110px; position: absolute; background-color: gold; top: 5px"></div>
 
 <div class="column-rule" style="left: 52px; height: 45px; top: 2px"></div>
 <div class="column-rule" style="left: 225px;"></div>
 <div class="column-rule" style="left: 398px; height: 18px;"></div>
 
-<div style="left: 175px; height: 10px; width: 110px; position: absolute; background-color: gold; top: 5px"></div>
-
 <div class="multi-column">
     <div id="flexbox">
         <div class="items">One</div>

css/css-gaps/flex/fragmentation/flex-gap-decorations-fragmentation-006-ref.html

@@ -47,11 +47,11 @@
     }
 </style>
 
-<div class="column-rule" style="top: 2px; left: 47px;"></div>
-<div class="column-rule" style="top: 0px; left: 220px;"></div>
+<div class="row-rule" style="left: 2px; top: 42px"></div>
+<div class="row-rule" style="left: 175px; top: 30px"></div>
 
-<div class="row-rule" style="left: 7px; top: 42px"></div>
-<div class="row-rule" style="left: 180px; top: 30px"></div>
+<div class="column-rule" style="top: 2px; left: 42px;"></div>
+<div class="column-rule" style="top: 0px; left: 215px;"></div>
 
 <div class="multi-column">
     <div id="flexbox">