Fix AnimationKeyframeEffectModel.MultipleProperties to not depend on the order of CSS property IDs

No functional change.

BUG=249451
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/111683005

git-svn-id: svn://svn.chromium.org/blink/trunk@164085 bbb929c8-8fbe-4397-9dbb-9b2b20218538