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This also required fixing the relayout when the scrollbar changes, which fixes
css3/flexbox/scrollbars-auto.html (and avoids breaking two tests in
external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox)

Bug: 635619

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I790f0524b029a3c2340b6b9715c5b3529c75357f
Reviewed-on: https://chromium-review.googlesource.com/1162657
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582464}