imported/csswg-test/css-flexbox-1/css-flexbox-height-animation-stretch.html is flaky
imported/csswg-test/css-flexbox-1/css-flexbox-height-animation-stretch.html is flaky

This test sometimes flakes on most platforms.

TBR=tkent@chromium.org
NOTRY=true
BUG=571531

Review URL: https://codereview.chromium.org/1545553004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#366534}

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