Attempt to fix flakiness in some newly added animation tests.
    
    As per http://crbug.com/275263 iteration events are collapsed so you
    may get less iteration events than what you expect (when duration is short).
    The tests are now modified in the following way : run the animation once
    and catch the animationstart and animationend events, then run the animation
    again with the iteration-count to infinite and when the first iteration event
    arrives we check the result and finish the test. Setting the iteration count
    to infinite make sure that at some point an iteration event will arrive (at the
    end the test is just checking wether the events are dispatched or not).

BUG=284531

Review URL: https://chromiumcodereview.appspot.com/23691030

git-svn-id: svn://svn.chromium.org/blink/trunk@157234 bbb929c8-8fbe-4397-9dbb-9b2b20218538