CSS Animations: Add a helper script for building interpolation tests

The current animation-test-helper is prone to flakiness and impossible
to use across browsers. This change adds a new helper which make it
possible to write fast, non-flaky, cross-browser tests which exercise
interpolation logic.

Review URL: https://chromiumcodereview.appspot.com/23412003

git-svn-id: svn://svn.chromium.org/blink/trunk@156755 bbb929c8-8fbe-4397-9dbb-9b2b20218538