Create timing functions directly instead of using parser in KeyframeEffectModelTest

As the tests in KeyframeEffectModelTest are unit tests, it doesn't make
sense for them to use the parser to construct timing functions when
they can be constructed directly.

Review URL: https://codereview.chromium.org/552123003

git-svn-id: svn://svn.chromium.org/blink/trunk@181609 bbb929c8-8fbe-4397-9dbb-9b2b20218538