Update TestExpectations: gardening.

virtual/threaded/animations/transition-and-animation-2.html flaky on mac also.

BUG=248938
TEST=
TBR=zmo@chromium.org

Review URL: https://codereview.chromium.org/195893024

git-svn-id: svn://svn.chromium.org/blink/trunk@169290 bbb929c8-8fbe-4397-9dbb-9b2b20218538