Mark a test flaky

which is svg/custom/foreign-object-skew.svg.

TBR=pdr
BUG=268692

Review URL: https://codereview.chromium.org/22268007

git-svn-id: svn://svn.chromium.org/blink/trunk@155559 bbb929c8-8fbe-4397-9dbb-9b2b20218538