Fix CompositedLayerMapping::startTransition's handling of filters

This removes an invalid assert in CompositedLayerMapping::startTransition,
and makes startTransition respect the return value of
GraphicsLayer::addAnimation when adding a filter animation.

BUG=181613

Review URL: https://codereview.chromium.org/52713009

git-svn-id: svn://svn.chromium.org/blink/trunk@161167 bbb929c8-8fbe-4397-9dbb-9b2b20218538