[FlexNG] RelayoutWithNewRowSizes stack overflow

If we RelayoutIgnoringChildScrollbarChanges() and then
RelayoutWithNewRowSizes(), we would hit a stack overflow because
RelayoutWithNewRowSizes() doesn't set |ignore_child_scrollbar_changes_|
(which leads us to alternate between the two relayouts infinitely).

Setting |ignore_child_scrollbar_changes_| fixes the issue.

Bug: 1315004
Change-Id: I9a6e815b74a76e958bb5c2d3989dfdf01fad3c26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3582397
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#991063}