Web Animations API: Add layout tests for invalid keyframe combinations.

Add layout tests for:
 - keyframes with properties that are not enumerable
 - keyframes with explicit composite: 'add'
 - keyframe sets that implicitly generate composite: 'add' synthetic keyframes
 - keyframe sets that combine auto offsets and out-of-order offsets

BUG=367644

Review URL: https://codereview.chromium.org/255323002

git-svn-id: svn://svn.chromium.org/blink/trunk@172939 bbb929c8-8fbe-4397-9dbb-9b2b20218538