Web Animations API: Add layout tests for element.animate() keyframes

Adds layout tests for keyframes with the following configurations:
 - 3 keyframes with and without offsets
 - Keyframes offsets specified out of order
 - Explicit null offset value
 - Shared offset keyframes with overlapping/disjoint properties
 - Invalid keyframe offsets
 - Per keyframe easing

Fixes flaky test issues due to bug 367903 by running the tests
inside requestAnimationFrame().

BUG=367644

Review URL: https://codereview.chromium.org/253963002

git-svn-id: svn://svn.chromium.org/blink/trunk@177186 bbb929c8-8fbe-4397-9dbb-9b2b20218538