Web Animations CSS: Creating toXXXTimingFunction methods and converting Blink to use them.

The DEFINE_TIMING_FUNCTION_TYPE_CASTS macro checks types and is generally the
preferred method in blink now.

BUG=

Review URL: https://codereview.chromium.org/74793004

git-svn-id: svn://svn.chromium.org/blink/trunk@162278 bbb929c8-8fbe-4397-9dbb-9b2b20218538