Mark web-animations-api/player-cancel-finishes.html as flaky on Mac
Mark web-animations-api/player-cancel-finishes.html as flaky on Mac

BUG=445955
TBR=loislo

Review URL: https://codereview.chromium.org/831013002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@187823 bbb929c8-8fbe-4397-9dbb-9b2b20218538

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk