Gardening: Mark virtual/threaded/animations/timing-model.html as flaky for Debug builds

This test has been randomly failing for a while in Debug bots for different platforms,
so now adding [ Pass Failure ] expectations for it.

BUG=344367
TBR=eae,abinader,alph
NOTRY=true

Review URL: https://codereview.chromium.org/169823002

git-svn-id: svn://svn.chromium.org/blink/trunk@167311 bbb929c8-8fbe-4397-9dbb-9b2b20218538