Remove assert guarding against providing keyframes with 'add' composite.
This is now tested by KeyframeEffectModelBase::isReplaceOnly(), which
means that API-provided keyframes with add compositing throw an exception
rather than crashing.

Review URL: https://codereview.chromium.org/250843006

git-svn-id: svn://svn.chromium.org/blink/trunk@172850 bbb929c8-8fbe-4397-9dbb-9b2b20218538