Web Animations API: Fix keyframe easing in non-experimental Blink

The unprefix property animation-timing-function is currently enabled
behind an experimental flag.
BisonCSSParser::parseAnimationTimingFunctionValue() relies on this
property to trigger the code path for parsing timing functions.
Without experimental features enabled this method fails to parse.

This patch changes the method to use the transition-timing-function
property as it is not guarded behind any flags.

BUG=368946

Review URL: https://codereview.chromium.org/260953002

git-svn-id: svn://svn.chromium.org/blink/trunk@173082 bbb929c8-8fbe-4397-9dbb-9b2b20218538