CSS Animations: Make interpolation test helper work without testRunner

The interpolation test helper currently doesn't handle well when being run
without the test runner due to problems with the current animation
implementation.
This patch works around the problems in the implementation to still be
able to show meaningful results to the user.

Review URL: https://chromiumcodereview.appspot.com/23522062

git-svn-id: svn://svn.chromium.org/blink/trunk@158341 bbb929c8-8fbe-4397-9dbb-9b2b20218538