[LayoutNG] Fragment caching interaction with FlexNG appears to be fixed

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=virtual%2Flayout_ng_experimental%2Fcss3%2Fflexbox%2Fflexbox-width-with-overflow-auto.html

So we can remove this failure line from TestExpectations.

Change-Id: Id2934204139eaa9c55807256f31d0ffa45a11e36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573809Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652231}

third_party/blink/web_tests/TestExpectations

@@ -1481,8 +1481,6 @@ Bug(none) virtual/layout_ng_experimental/fragmentation/scrolling-contents-scroll
 
 # Flexbox in NG
 #
-# Fails due to fragment caching (?)
-crbug.com/591099 virtual/layout_ng_experimental/css3/flexbox/flexbox-width-with-overflow-auto.html [ Failure ]
 
 ### virtual/layout_ng_experimental/css3/flexbox/
 crbug.com/591099 virtual/layout_ng_experimental/css3/flexbox/auto-height-column-with-border-and-padding.html [ Failure ]