Improve keyframes-rule.html to avoid cascading failures

Currently this test repeatedly modifies and checks the properties of a small
number of CSS keyframe objects. This means that if one check fails, many of the
following checks fail too, and the output of the test is difficult to
interpret.

This change modifies the test to use a separate CSS keyframe object for each
check, so that failures don't cascade. No change in behavior or test coverage.

This is a preparatory change before fixing some aspects of the keyframe CSSOM
behavior.

R=apavlov

Review URL: https://codereview.chromium.org/109303004

git-svn-id: svn://svn.chromium.org/blink/trunk@164163 bbb929c8-8fbe-4397-9dbb-9b2b20218538