ElementAnimations should hold an ObservableList of AnimationPlayers.

      Before this, the AnimationPlayers were held in a linked list, and
modifying the list while iterating could cause undefined behavior.

BUG=631052
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2189813002
Cr-Commit-Position: refs/heads/master@{#410152}