Rebaseline: Mark svg/custom/animate-initial-pause-unpause.html flaky

BUG=350828
TBR=pdr

Review URL: https://codereview.chromium.org/189913013

git-svn-id: svn://svn.chromium.org/blink/trunk@168820 bbb929c8-8fbe-4397-9dbb-9b2b20218538